Use thread-local is_gc_marking flags for the CC collector.

The currently global is_marking flag is used to check if the read
barrier slow path needs to be taken for GC roots access. Changing it
to a thread-local flag simplifies the fast path check and makes it
easier to do it in assembly code. It also solves the issue that we
need to avoid accessing the global flag during startup before the heap
or the collector object isn't allocated and initialized.

Bug: 12687968
Change-Id: Ibf0dca12f400bf3490188b12dfe96c7de30583e0