Ensure instruction cache is flushed in compiler tests with Clang.

Invoke __builtin___clear_cache with Clang in
art::CommonCompilerTest::MakeExecutable.  This fixes
"Illegal instruction" failures in codegen_test on angler.

Factor this logic in a function and use it in
art::jit::JitCodeCache::CommitCodeInternal too.

Bug: 26786447
Change-Id: I81fa056c8d7c6bf2168c9340deeab82231cb1084