Add target armv7s-darwin-gcc.

Really just armv7. This is a convenience target intended to make iOS
development with libvpx easier. Xcode projects with default settings
will fail to build when a framework lacks armv7s support when targetting
iOS7.

Change-Id: I7eb80d52eec25501febc0d2c3c0b4ed964b8ed5b
3 files changed