Prevent ArtMethod clearing race

There was race condition where the GC would be visiting the methods
of a class while the class linker overwrote the contents with 0xFE
when copying to a new array.

Since the GC is holding the class table lock at this time, we can
use this lock in the class linker to prevent the race.

Bug: 28699001
Change-Id: I91ca2406ef723e7be69cd1c53f4bafa2e5f63657