Use stack allocated context for VisitRoots

Prevents allocation of contexts for threads that do not throw
exceptions.

Saves up to 544 per thread on arm64.

Bug: 36457259

Test: mm test-art-host on with N6P target (build covers arm32, arm64)
Change-Id: I782a1115d624edcc1279fa2317d026408dfa8a10
3 files changed