Stacks grow downward on x86_64

I exchanged [rbp-0x20] with [rbp+0x20], making the JIT write the
frame number outside its stack space. On OSX, this caused a crash,
but unfortunately it was masked by alignment on linux/windows and
under debuggers.

src/jit/emit_x64.dasc

@@ -277,7 +277,7 @@ const unsigned int MVM_jit_num_globals(void) {
 | callp &MVM_exception_throw_adhoc;
 |.endmacro
 
-|.define FRAME_NR, dword [rbp+0x20]
+|.define FRAME_NR, dword [rbp-0x20]
 
 /* A function prologue is always the same in x86 / x64, because
  * we do not provide variable arguments, instead arguments are provided