disable GCC and 32-bit builds on Mac

It used to be pretty easy to use Homebrew GCC on Mac with skcms,
but I keep running into issues that have me disable these builds
locally.  (They're already disabled on the bots... see bot.py.)

32-bit x86 builds are deprecated on Mac and give a slightly annoying
warning.  I don't mind flipping it back on manually if I ever need to
try it out.

Change-Id: Ibf4d480bb2ee7e48fdbdaa31c816a4cefee74307
Reviewed-on: https://skia-review.googlesource.com/c/162103
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed