Work around issues in MinGW-64's setjmp/longjmp support.

It's hard to avoid the conclusion that there is something wrong with
setjmp/longjmp on MinGW-64, as we have seen failures come and go after
entirely-unrelated-looking changes in our own code.  Other projects
such as Ruby have given up and started using gcc's setjmp/longjmp
builtins on that platform; this patch just follows that lead.

Note that this is a pretty fundamental ABI break for functions
containining either setjmp or longjmp, so we can't really consider
a back-patch.

Per reports from Regina Obe and Heath Lord, as well as recent failures
on buildfarm member walleye, and less-recent failures on fairywren.

Juan José Santamaría Flecha

Discussion: https://postgr.es/m/000401d716a0$1ed0fc70$5c72f550$@pcorp.us
Discussion: https://postgr.es/m/CA+BEBhvHhM-Bn628pf-LsjqRh3Ang7qCSBG0Ga+7KwhGqrNUPw@mail.gmail.com
Discussion: https://postgr.es/m/f1caef93-9640-022e-9211-bbe8755a56b0@2ndQuadrant.com

src/include/c.h

@@ -1335,14 +1335,21 @@ extern unsigned long long strtoull(const char *str, char **endptr, int base);
 
 /*
  * When there is no sigsetjmp, its functionality is provided by plain
- * setjmp. Incidentally, nothing provides setjmp's functionality in
- * that case.  We now support the case only on Windows.
+ * setjmp.  We now support the case only on Windows.  However, it seems
+ * that MinGW-64 has some longstanding issues in its setjmp support,
+ * so on that toolchain we cheat and use gcc's builtins.
  */
 #ifdef WIN32
+#ifdef __MINGW64__
+typedef intptr_t sigjmp_buf[5];
+#define sigsetjmp(x,y) __builtin_setjmp(x)
+#define siglongjmp __builtin_longjmp
+#else							/* !__MINGW64__ */
 #define sigjmp_buf jmp_buf
 #define sigsetjmp(x,y) setjmp(x)
 #define siglongjmp longjmp
-#endif
+#endif							/* __MINGW64__ */
+#endif							/* WIN32 */
 
 /* EXEC_BACKEND defines */
 #ifdef EXEC_BACKEND