[Kernel] Remove explicit SHIM calling convention

Shim exports are called from GuestToHostThunk which dictates the calling
convention. The default system calling convention is different depending
on OS (Windows vs. everything else) and architecture. PR #1339 addresses
this for x64 Linux.
There is no reason for explicit `__cdecl`. Also, it is not available in
GCC. We could use `__attribute__((ms_abi))` or
`__attribute__((sysv_abi))` but that just adds complexity.

src/xenia/kernel/util/shim_utils.h

@@ -29,7 +29,7 @@ namespace kernel {
 
 using PPCContext = xe::cpu::ppc::PPCContext;
 
-#define SHIM_CALL void __cdecl
+#define SHIM_CALL void
 #define SHIM_SET_MAPPING(library_name, export_name, shim_data) \
   export_resolver->SetFunctionMapping(                         \
       library_name, ordinals::export_name,                     \