GN build fixes, mostly for Mac.

This hooks up detection for the "-arch" flag on Mac to set the GYP "ARCH" xcode variable. GN then removes the -arch argument from the compiler args, since GYP will then re-add it based on the ARCH value. Previously, not doing this resulting in mutliple "-arch" arguments to the compiler since GYP would always insert its own.

Disables some warnings on Windows for the re2 target to match the GYP build. The third warning (4018) that the GYP build sets is disabled globally so there's no need to do it for this target.

Hooks up some iOS SDK stuff.
BUG=336667
TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/141433015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247206 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247206