Add armv7s-ios cmake toolchain.

This is used at cmake generation time via a command line like this:
$ cmake path/to/aom -DCMAKE_TOOLCHAIN_FILE=path/to/aom/build/cmake/toolchain/armv7-ios.cmake

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: Iadacc32c43bc23e0f670b88e3c1563c44319945c
4 files changed