win64: add tiny unwind data for setjmp/longjmp

This enables native unwind semantics with longjmp on
win64 by putting an entry into the .pdata section for
each compiled fuction.

Also, the function now use a fixed stack and store arguments
into X(%rsp) rather than using push.

win32/include/_mingw.h

@@ -58,7 +58,7 @@ typedef unsigned __int64 uintptr_t;
 #define _AMD64_ 1
 #define __x86_64 1
 #define USE_MINGW_SETJMP_TWO_ARGS
-#define mingw_getsp tinyc_getsp
+#define mingw_getsp tinyc_getbp
 #else
 typedef __int32 intptr_t;
 typedef unsigned __int32 uintptr_t;