Fix PATH and EXE quoting for Windows ARM64 builds

The PATH change we make is the only thing that needs Windows style
paths (and won't be fixed by GN). So, undo the other backslashes,
and fix a minor error in the placement of the quote.

Bug: skia:8569
Change-Id: I4feecaac96a4cf4d221a93e852a63f597e25e892
Reviewed-on: https://skia-review.googlesource.com/c/175825
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

Author: brianosman

gn/toolchain/BUILD.gn

@@ -33,12 +33,12 @@ toolchain("msvc") {
 
   if (msvc == 2015) {
     if (target_cpu == "x86") {
-      bin = "$win_vc\\bin"
+      bin = "$win_vc/bin"
     } else {
-      bin = "$win_vc\\bin\\amd64"
+      bin = "$win_vc/bin/amd64"
     }
   } else {
-    bin = "$win_vc\\Tools\\MSVC\\$win_toolchain_version\\bin\\HostX64\\$target_cpu"
+    bin = "$win_vc/Tools/MSVC/$win_toolchain_version/bin/HostX64/$target_cpu"
   }
 
   env_setup = ""
@@ -48,7 +48,7 @@ toolchain("msvc") {
     env_setup = "cmd /c $win_sdk/bin/SetEnv.cmd /x86 && "
   } else if (target_cpu == "arm64") {
     # ARM64 compiler is incomplete - it relies on DLLs located in the host toolchain directory.
-    env_setup = "cmd /C \"set PATH=%PATH%;$win_vc\\Tools\\MSVC\\$win_toolchain_version\\bin\\HostX64\\x64\" && "
+    env_setup = "cmd /C set \"PATH=%PATH%;$win_vc\\Tools\\MSVC\\$win_toolchain_version\\bin\\HostX64\\x64\" && "
   }
 
   cl_m32_flag = ""
@@ -65,9 +65,9 @@ toolchain("msvc") {
       lib = "\"$clang_win/bin/lld-link.exe\" /lib"
       link = "\"$clang_win/bin/lld-link.exe\""
     } else {
-      cl = "$clang_win/bin/clang-cl"
-      lib = "$clang_win/bin/lld-link /lib"
-      link = "$clang_win/bin/lld-link"
+      cl = "\"$clang_win/bin/clang-cl\""
+      lib = "\"$clang_win/bin/lld-link\" /lib"
+      link = "\"$clang_win/bin/lld-link\""
     }
   } else {
     cl = "\"$bin/cl.exe\""