[Bugpoint] Use 'CC' instead of 'GCC' for variable naming.

[Bugpoint] Use 'CC' instead of 'GCC' for variable naming.

We now use clang by default and fallback to gcc when requested.
With this commit, names reflect reality. No functional change
intended.

Discussed with: Rafael Espindola.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250321 91177308-0d34-0410-b5e6-96231b3b80d8