Speed up "gn gen" by ~65ms for non-mac host_os

By skipping an xcode-specific exec_script().
Discovered via "gn gen --tracelog=gn.trace"

Bug: 748851
Change-Id: I7d7d1f636e90d4842758d94a1b327622bb9f5edc
Reviewed-on: https://chromium-review.googlesource.com/609163
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493431}

build_overrides/build.gni

@@ -48,7 +48,7 @@ declare_args() {
   use_system_xcode = ""
 }
 
-if (use_system_xcode == "") {
+if (host_os == "mac" && use_system_xcode == "") {
   _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
                         [ target_os ],
                         "value")