Use entrypoint switching on x86 & x86-64 for GC root read barriers.

For consistency reason (with the ARM and ARM64 implementations),
check the read barrier marking entrypoint
(`Thread::Current()->pReadBarrierMarkReg ## root.reg()`)
instead of `Thread::Current()->GetIsGcMarking()` to decide whether
to mark a GC root.

This change should have no impact on the performance or the
size of the generated code.

Test: m test-art-host
Bug: 32638713
Change-Id: Ifd71312992fdfd6067447cccb7d95860f3771b57