Improve art_quick_resolve_string for arm64 CC

Check gc_is_marking instead of the mark bit first, this makes it
that we don't go slow path if the GC is not running.

Also reduced the code by a few instructions.

EAAC CC: ~2000 -> 1256

Test: test-art-target-run-test -j4

Bug: 20323084
Change-Id: I57a1f7a52f1909e2e5dd1b2cfd2612b4a642fe37