CMakeLists: Specify -Wextra on linux builds

Allows reporting more cases where logic errors may exist, such as
implicit fallthrough cases, etc.

We currently ignore unused parameters, since we currently have many
cases where this is intentional (virtual interfaces).

While we're at it, we can also tidy up any existing code that causes
warnings. This also uncovered a few bugs as well.

src/core/hle/service/audio/audren_u.cpp

@@ -129,7 +129,7 @@ private:
         LOG_DEBUG(Service_Audio, "called. rendering_time_limit_percent={}",
                   rendering_time_limit_percent);
 
-        ASSERT(rendering_time_limit_percent >= 0 && rendering_time_limit_percent <= 100);
+        ASSERT(rendering_time_limit_percent <= 100);
 
         IPC::ResponseBuilder rb{ctx, 2};
         rb.Push(RESULT_SUCCESS);

src/core/hle/service/filesystem/filesystem.cpp

@@ -451,7 +451,8 @@ FileSys::SaveDataSize FileSystemController::ReadSaveDataSize(FileSys::SaveDataTy
 
         if (res != Loader::ResultStatus::Success) {
             FileSys::PatchManager pm{system.CurrentProcess()->GetTitleID()};
-            auto [nacp_unique, discard] = pm.GetControlMetadata();
+            const auto metadata = pm.GetControlMetadata();
+            const auto& nacp_unique = metadata.first;
 
             if (nacp_unique != nullptr) {
                 new_size = {nacp_unique->GetDefaultNormalSaveSize(),

src/core/hle/service/filesystem/fsp_srv.cpp

@@ -575,6 +575,7 @@ private:
                                 0,
                                 user_id->GetSize(),
                                 {},
+                                {},
                             });
 
                             continue;
@@ -595,6 +596,7 @@ private:
                                 stoull_be(title_id->GetName()),
                                 title_id->GetSize(),
                                 {},
+                                {},
                             });
                         }
                     }
@@ -619,6 +621,7 @@ private:
                                 stoull_be(title_id->GetName()),
                                 title_id->GetSize(),
                                 {},
+                                {},
                             });
                         }
                     }

src/core/hle/service/time/time_zone_manager.cpp

@@ -309,7 +309,7 @@ static bool ParsePosixName(const char* name, TimeZoneRule& rule) {
         offset = GetTZName(name, offset);
         std_len = offset;
     }
-    if (!std_len) {
+    if (std_len == 0) {
         return {};
     }
     if (!GetOffset(name, offset, std_offset)) {
@@ -320,7 +320,7 @@ static bool ParsePosixName(const char* name, TimeZoneRule& rule) {
     int dest_len{};
     int dest_offset{};
     const char* dest_name{name + offset};
-    if (rule.chars.size() < char_count) {
+    if (rule.chars.size() < std::size_t(char_count)) {
         return {};
     }
 
@@ -343,7 +343,7 @@ static bool ParsePosixName(const char* name, TimeZoneRule& rule) {
             return {};
         }
         char_count += dest_len + 1;
-        if (rule.chars.size() < char_count) {
+        if (rule.chars.size() < std::size_t(char_count)) {
             return {};
         }
         if (name[offset] != '\0' && name[offset] != ',' && name[offset] != ';') {
@@ -414,7 +414,7 @@ static bool ParsePosixName(const char* name, TimeZoneRule& rule) {
                 if (is_reversed ||
                     (start_time < end_time &&
                      (end_time - start_time < (year_seconds + (std_offset - dest_offset))))) {
-                    if (rule.ats.size() - 2 < time_count) {
+                    if (rule.ats.size() - 2 < std::size_t(time_count)) {
                         break;
                     }
 
@@ -609,7 +609,7 @@ static bool ParseTimeZoneBinary(TimeZoneRule& time_zone_rule, FileSys::VirtualFi
     }
 
     const u64 position{(read_offset - sizeof(TzifHeader))};
-    const std::size_t bytes_read{vfs_file->GetSize() - sizeof(TzifHeader) - position};
+    const s64 bytes_read = s64(vfs_file->GetSize() - sizeof(TzifHeader) - position);
     if (bytes_read < 0) {
         return {};
     }
@@ -621,11 +621,11 @@ static bool ParseTimeZoneBinary(TimeZoneRule& time_zone_rule, FileSys::VirtualFi
     std::array<char, time_zone_name_max + 1> temp_name{};
     vfs_file->ReadArray(temp_name.data(), bytes_read, read_offset);
     if (bytes_read > 2 && temp_name[0] == '\n' && temp_name[bytes_read - 1] == '\n' &&
-        time_zone_rule.type_count + 2 <= time_zone_rule.ttis.size()) {
+        std::size_t(time_zone_rule.type_count) + 2 <= time_zone_rule.ttis.size()) {
         temp_name[bytes_read - 1] = '\0';
 
         std::array<char, time_zone_name_max> name{};
-        std::memcpy(name.data(), temp_name.data() + 1, bytes_read - 1);
+        std::memcpy(name.data(), temp_name.data() + 1, std::size_t(bytes_read - 1));
 
         TimeZoneRule temp_rule;
         if (ParsePosixName(name.data(), temp_rule)) {

src/core/hle/service/vi/vi.cpp

@@ -101,8 +101,8 @@ public:
     }
 
     std::u16string ReadInterfaceToken() {
-        u32 unknown = Read<u32_le>();
-        u32 length = Read<u32_le>();
+        [[maybe_unused]] const u32 unknown = Read<u32_le>();
+        const u32 length = Read<u32_le>();
 
         std::u16string token{};