Switch from GCC to Clang. BUG=18470793 Change-Id: I850fdd6314f0932c052e334f60ad4e272de51398 Reviewed-on: https://swiftshader-review.googlesource.com/1463 Reviewed-by: Nicolas Capens <nicolascapens@google.com> Tested-by: Nicolas Capens <nicolascapens@google.com>