core: LOCAL_GCC support & experimental => development type

Summary:
  o Implement LOCAL_GCC support for disabling utilizing
    cfx clang
  o Always find GCC's libgcov.a and libgcc.a
  o Switch to clang by default globally
  o Disable toolchain build for now until full llvm
    toolchain is integrated inline

Test Plan: finish full platform, test all devices, and merge

Reviewers: randall.rushing

CC: samuaz

Differential Revision: http://review.codefirex.com/D613