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/Make_mvc.mak b/src/Make_mvc.mak
index 028277a..639ae2f 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -222,8 +222,12 @@
 !  endif
 ! else  # !CPU
 CPU = i386
-!  if !defined(PLATFORM) && defined(TARGET_CPU)
+!  ifndef PLATFORM
+!   ifdef TARGET_CPU
 PLATFORM = $(TARGET_CPU)
+!   elseif defined(VSCMD_ARG_TGT_ARCH)
+PLATFORM = $(VSCMD_ARG_TGT_ARCH)
+!   endif
 !  endif
 !  ifdef PLATFORM
 !   if ("$(PLATFORM)" == "x64") || ("$(PLATFORM)" == "X64")
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,