Remove the Darwin_Generic_GCC toolchain.

This is currently unused by any test. The code path would still be hit
by clang on ppc, but

* PPC has not been supported on current versions of OS X
* A port of current clang to older OS X on ppc should be using
toolchains::DarwinClang.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195585 91177308-0d34-0410-b5e6-96231b3b80d8