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;

win32/lib/chkstk.S

@@ -57,11 +57,19 @@ P0:
     jmp     *8(%rax)
 
 /* ---------------------------------------------- */
-/* avoid msvcrt unwind stuff for setjmp/longjmp */
+/* setjmp/longjmp support */
 
-.globl tinyc_getsp
-tinyc_getsp:
+.globl tinyc_no_getbp
+tinyc_no_getbp:
+    .byte 0x90
+
+.globl tinyc_getbp
+tinyc_getbp:
     xor %rax,%rax
+    cmp %al,tinyc_no_getbp(%rax)
+    je t1
+    mov %rbp,%rax
+t1:
     ret
 
 /* ---------------------------------------------- */