patch 8.1.0941: macros for MS-Windows are inconsistent

Problem:    Macros for MS-Windows are inconsistent, using "32", "3264 and
            others.
Solution:   Use MSWIN for all MS-Windows builds.  Use FEAT_GUI_MSWIN for the
            GUI build. (Hirohito Higashi, closes #3932)
diff --git a/src/memfile.c b/src/memfile.c
index 1a52734..1827bcf 100644
--- a/src/memfile.c
+++ b/src/memfile.c
@@ -621,7 +621,7 @@
 		status = FAIL;
 	}
 #endif
-#ifdef WIN32
+#ifdef MSWIN
 	if (_commit(mfp->mf_fd))
 	    status = FAIL;
 #endif
@@ -1265,7 +1265,7 @@
 	 * try to open the file
 	 */
 	flags |= O_EXTRA | O_NOFOLLOW;
-#ifdef WIN32
+#ifdef MSWIN
 	/* Prevent handle inheritance that cause problems with Cscope
 	 * (swap file may not be deleted if cscope connection was open after
 	 * the file) */