Remove support for building with llvm-gcc

Now that Apple no longer ships llvm-gcc, and Homebrew offers several
alternatives to clang for users who need GCC compilers (apple-gcc42,
GNU GCCs from homebrew/versions), there's no longer a reason to
support building with llvm-gcc.

This commit removes llvm-gcc and its related options. Platforms that
defaulted to llvm-gcc will now default to Apple's gcc-4.2 or clang,
depending on availability.

fails_with(:llvm) is now a no-op, to retain compatibility.