lib/clang: clean out old llvm/clang Sparc target remnants

The WITH_LLVM_TARGET_ENABLE_SPARC option was removed a long time ago,
but some ifdefs were still laying around, so clean them up.

PR: 276104
MFC after: 3 days

(cherry picked from commit 6f444019009a55aac18d18054d154155fbf606c9)