Fix WITHOUT_CLANG build.

This decouples MK_LLVM_TARGET_ALL from MK_CLANG. It is fine if
LLVM_TARGET_* are set even if MK_CLANG is disabled. It never
made sense to depend MK_LLVM_TARGET_* to MK_CLANG (which I did
in r335706).

PR: 240507
Reported by: kevans, swills
MFC after: 2 weeks