windows: replace remaining __MINGW32__ instances with _WIN32

R=rsc, brainman
CC=golang-dev
https://golang.org/cl/4146041

include/libc.h

@@ -291,7 +291,7 @@ extern	char*	getgoarch(void);
 extern	char*	getgoroot(void);
 extern	char*	getgoversion(void);
 
-#ifdef __MINGW32__
+#ifdef _WIN32
 struct timespec {
 	int tv_sec;
 	long tv_nsec;

include/u.h

@@ -78,7 +78,7 @@ extern "C" {
 #define _NEEDUINT 1
 #define _NEEDULONG 1
 
-#ifdef __MINGW32__
+#ifdef _WIN32
 typedef jmp_buf sigjmp_buf;
 #endif
 typedef long p9jmp_buf[sizeof(sigjmp_buf)/sizeof(long)];
@@ -139,7 +139,7 @@ typedef long p9jmp_buf[sizeof(sigjmp_buf)/sizeof(long)];
 #	undef _NEEDUSHORT
 #	undef _NEEDUINT
 #	undef _NEEDULONG
-#elif defined(__MINGW32__)
+#elif defined(_WIN32)
 #else
 	/* No idea what system this is -- try some defaults */
 #	include <pthread.h>
@@ -208,7 +208,7 @@ typedef u64int uint64;
  */
 #if defined(__GNUC__)
 #	undef strcmp	/* causes way too many warnings */
-#	if __GNUC__ >= 4 || (__GNUC__==3 && !defined(__APPLE_CC__) && !defined(__MINGW32__))
+#	if __GNUC__ >= 4 || (__GNUC__==3 && !defined(__APPLE_CC__) && !defined(_WIN32))
 #		undef AUTOLIB
 #		define AUTOLIB(x) int __p9l_autolib_ ## x __attribute__ ((weak));
 #		undef AUTOFRAMEWORK