Fix warnings in core and common

src/core/file_sys/file_sdmc.cpp

@@ -53,7 +53,7 @@ size_t File_SDMC::Write(const u64 offset, const u32 length, const u32 flush, con
 }
 
 size_t File_SDMC::GetSize() const {
-    return file->GetSize();
+    return static_cast<size_t>(file->GetSize());
 }
 
 bool File_SDMC::Close() const {

src/core/mem_map_funcs.cpp

@@ -287,7 +287,7 @@ void Write64(const VAddr addr, const u64 data) {
 }
 
 void WriteBlock(const VAddr addr, const u8* data, const size_t size) {
-    int offset = 0;
+    u32 offset = 0;
     while (offset < (size & ~3)) {
         Write32(addr + offset, *(u32*)&data[offset]);
         offset += 4;