[ARM] Emit llvm.clear_cache for __clear_cache() (#223398)
It looks like the ARM-specific __clear_cache() builtin emits different
IR than the generic __builtin___clear_cache() builtin (added later),
which uses the llvm.clear_cache intrinsic (which will typically lower to
a __clear_cache libcall, of course).
Use llvm.clear_cache for __clear_cache() as well, for consistency and to
slightly simplify the code.
