ART: remove gcc cruft from the makefiles

ART only builds with clang now, delete all logic to determine gcc
vs. clang, and move all LOCAL_CLANG_CFLAGS into LOCAL_CFLAGS.

Test: mma, make test-art-host, make test-art-target
Change-Id: I578615233ad3e71af18afb47bb11f3be8417216c
10 files changed