Fix PATH when running MSVC for ARM64 target

The ARM64 MSVC toolchain only includes some of the executables. Other
functionality is in DLLs that are located in the host (x64) toolchain
directory. Hopefully MS fixes this at some point.

Bug: skia:8569
Change-Id: I1a96c63c9bdcf656eb2d84b81636d54304766c20
Reviewed-on: https://skia-review.googlesource.com/c/175821
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/gn/BUILDCONFIG.gn b/gn/BUILDCONFIG.gn
index 031839d..5a1c4d0 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -137,12 +137,12 @@
   # By default we look for 2017 (Pro & Community), then 2015. If MSVC is installed in a
   # non-default location, you can set win_vc to inform us where it is.
   vc_2017_pro_default =
-      "C:/Program Files (x86)/Microsoft Visual Studio/2017/Professional/VC"
+      "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Professional\\VC"
   vc_2017_com_default =
-      "C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC"
+      "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Community\\VC"
   vc_2017_bt_default =
-      "C:/Program Files (x86)/Microsoft Visual Studio/2017/BuildTools/VC"
-  vc_2015_default = "C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC"
+      "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\BuildTools\\VC"
+  vc_2015_default = "C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC"
 
   if (win_vc == "") {
     if ("True" == exec_script("//gn/checkdir.py",