Change 64 bit atomic counter to size_t

There was a 64 bit cas that was showing up as very expensive on 32
bit X86. The counter was for Region::objects_allocated_. This change
moves it to be size_t.

X86 GC time: 13.6s -> 11.1s
X86 perf: ConcurrentCopying::Copy 14.07% -> 3.73% of samples

Benchmark score unmeasured.

Bug: 12687968

Test: test-art-host

Change-Id: I9db969fc9f2649a14ff755ab4a43d287c2b47958