kernel/svc: Implement svcGetThreadContext()

Now that we have all of the rearranging and proper structure sizes in
place, it's fairly trivial to implement svcGetThreadContext(). In the
64-bit case we can more or less just write out the context as is, minus
some minor value sanitizing. In the 32-bit case we'll need to clear out
the registers that wouldn't normally be accessible from a 32-bit
AArch32 exectuable (or process).

src/core/hle/kernel/svc_wrap.h

@@ -64,6 +64,11 @@ void SvcWrap() {
     FuncReturn(func(Param(0), (s32)Param(1)).raw);
 }
 
+template <ResultCode func(u64, u32)>
+void SvcWrap() {
+    FuncReturn(func(Param(0), static_cast<u32>(Param(1))).raw);
+}
+
 template <ResultCode func(u64*, u64)>
 void SvcWrap() {
     u64 param_1 = 0;