[x64] Fix stack manipulation in EmitHostToGuestThunk

To access the stack at adresses higher than the stack pointer, it
must be moved down (subtracted) before the accesses. If not, the stack
pointer still points to the previous stack frame, corrupting it.

This fixes a crash on Linux when compiled in release mode with clang.

Author: amessier

src/xenia/cpu/backend/x64/x64_backend.cc

@@ -434,10 +434,10 @@ HostToGuestThunk X64ThunkEmitter::EmitHostToGuestThunk() {
   code_offsets.prolog = getSize();
 
   // rsp + 0 = return address
+  sub(rsp, stack_size);
   mov(qword[rsp + offsetof(StackLayout::Thunk, arg_temp[2])], GetNativeReg(2));
   mov(qword[rsp + offsetof(StackLayout::Thunk, arg_temp[1])], GetNativeReg(1));
   mov(qword[rsp + offsetof(StackLayout::Thunk, arg_temp[0])], GetNativeReg(0));
-  sub(rsp, stack_size);
 
   code_offsets.prolog_stack_alloc = getSize();
   code_offsets.body = getSize();
@@ -454,10 +454,10 @@ HostToGuestThunk X64ThunkEmitter::EmitHostToGuestThunk() {
 
   code_offsets.epilog = getSize();
 
-  add(rsp, stack_size);
   mov(GetNativeReg(0), qword[rsp + offsetof(StackLayout::Thunk, arg_temp[0])]);
   mov(GetNativeReg(1), qword[rsp + offsetof(StackLayout::Thunk, arg_temp[1])]);
   mov(GetNativeReg(2), qword[rsp + offsetof(StackLayout::Thunk, arg_temp[2])]);
+  add(rsp, stack_size);
   ret();
 
   code_offsets.tail = getSize();