Implements ARCHS filtering for all generator on OS X

Implements ARCHS filtering by VALID_ARCHS and variable expansion, like
$(ARCHS_STANDARD), in XcodeSettings.GetActiveArchs so it is available
for all generators and all SDKROOT (macosx, iphoneos, iphonesimulator)
instead of being restricted to iOS projects.

BUG=None
R=mark@chromium.org

Review URL: https://codereview.chromium.org/211023003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1884 78cadc50-ecff-11dd-a971-7dbc132099af