This CL implements ldrex, ldrexb, ldrexh, strex, strexb, and strexh in the
Simulator. These instructions provide "exclusive" access, which provides mutual
exclusion for concurrent threads of execution.

The ARM specification gives some leeway to implementors, but essentially
describes each processor as having Local Monitor and Global Monitor. The Local
Monitor is used to check the exclusivity state without having to synchronize
with other processors. The Global Monitor is shared between processors. We
model both to make it easier to match behavior with the spec.

When running with multiple OS threads, each thread has its own isolate, and
each isolate has its own Simulator. The Local Monitor is stored directly on the
Simulator, and the Global Monitor is stored as a lazy singleton. The Global
Monitor maintains a linked-list of all Simulators.

All loads/stores (even non-exclusive) are guarded by the Global Monitor's mutex.

BUG=v8:4614

Review-Url: https://codereview.chromium.org/2006183004
Cr-Commit-Position: refs/heads/master@{#42481}