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.