cmd/dist: use vfprintf on Windows, same as on Unix

Apparently, the Windows routines sometimes fail to generate output.
Copy the Unix stdio-based implementations instead.

Suggested by Pietro Gagliardi in CL 65280043 but that CL
seems to have been abandoned.

Fixes #7242.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/71550044

src/cmd/dist/windows.c

@@ -840,34 +840,20 @@ void
 xprintf(char *fmt, ...)
 {
 	va_list arg;
-	char *p;
-	DWORD n, w;
-
+	
 	va_start(arg, fmt);
-	n = vsnprintf(NULL, 0, fmt, arg);
-	p = xmalloc(n+1);
-	vsnprintf(p, n+1, fmt, arg);
+	vprintf(fmt, arg);
 	va_end(arg);
-	w = 0;
-	WriteFile(GetStdHandle(STD_OUTPUT_HANDLE), p, n, &w, 0);
-	xfree(p);
 }
 
 void
 errprintf(char *fmt, ...)
 {
 	va_list arg;
-	char *p;
-	DWORD n, w;
-
+	
 	va_start(arg, fmt);
-	n = vsnprintf(NULL, 0, fmt, arg);
-	p = xmalloc(n+1);
-	vsnprintf(p, n+1, fmt, arg);
+	vfprintf(stderr, fmt, arg);
 	va_end(arg);
-	w = 0;
-	WriteFile(GetStdHandle(STD_ERROR_HANDLE), p, n, &w, 0);
-	xfree(p);
 }
 
 int