For support of new GCC v4, obtain the full and major versions of the

For support of new GCC v4, obtain the full and major versions of the
llvm-gcc command. This will help distinguish which version is being used
so the makefiles can accommodate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27461 91177308-0d34-0410-b5e6-96231b3b80d8