patch 8.1.0951: using WIN64 even though it is never defined

Problem:    Using WIN64 even though it is never defined.
Solution:   Only use _WIN64. (Ken Takata, closes #3997)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index e94d875..7f50555 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6158,7 +6158,7 @@
 #if defined(UNIX) && (defined(__CYGWIN32__) || defined(__CYGWIN__))
 	"win32unix",
 #endif
-#if defined(WIN64) || defined(_WIN64)
+#ifdef _WIN64
 	"win64",
 #endif
 #ifdef EBCDIC