[mac] Create symlinks to same location as iOS

The script build/config/mac/sdk_info.py expects a relative path for
`--create_symlinks_at` parameter. The path as constructed was at
`$root_out_dir/$root_out_dir_relative_to_src/sdk/xcode_links` while
it is at `$root_out_dir/sdk/xcode_links` for iOS.

The path used on iOS makes more sense (and was necessary to fix a
bug, see https://crrev.com/c/2102544) so use the same path on mac.

Bug: 1015730
Change-Id: If234d3f598dc7abdd70d78a8af8d3d140b47cadc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2501742
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821205}

build/config/mac/mac_sdk.gni

@@ -81,7 +81,7 @@ if (use_system_xcode && ios_use_goma_rbe) {
   sdk_info_args += [
     "--get_sdk_info",
     "--create_symlink_at",
-    rebase_path("$root_build_dir/sdk/xcode_links", "//"),
+    "sdk/xcode_links",
   ]
 }
 sdk_info_args += [ mac_sdk_name ]