Hopefully fix MVM_jit_emit_runcode on Windows

Windows requires structs and other aggregates to be put into caller allocated
memory and a pointer to this memory passed as an ordinary argument. On POSIX
we put the struct's fields individually onto the stack. Since the stack is
memory and as such guaranteed to be available until the caller exits, we can
use it for Windows as well. So all that should be needed is that pointer.

Completely untested, as I don't have Windows available.

src/jit/x64/emit.dasc

@@ -3065,6 +3065,10 @@ void MVM_jit_emit_runcode(MVMThreadContext *tc, MVMJitCompiler *compiler, MVMJit
     emit_stack_arg(tc, compiler, jg, 8, 8);
     | lea TMP6, [<5];
     emit_stack_arg(tc, compiler, jg, 8, 16);
+    |.if WIN32
+        /* Pass a pointer to the stack allocated struct as ordinary argument */
+        | mov ARG2, rsp;
+    |.endif
     | mov FUNCTION, qword WORK[runcode->code_register];
     | mov FUNCTION, CFUNCTION:FUNCTION->body.func;
     | call FUNCTION