patch 9.0.0857: selecting MSVC 2017 does not set $PLATFORM

Problem:    Selecting MSVC 2017 does not set $PLATFORM.
Solution:   Use $VSCMD_ARG_TGT_ARCH. (Ken Takata, closes #11485)
diff --git a/src/version.c b/src/version.c
index 50f6709..19f0692 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    857,
+/**/
     856,
 /**/
     855,