Switch GYP to use LLVM-GCC when using Xcode.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/12228004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13600 ce2b1a6d-e550-0410-aec6-3dcde31c8c00