Fix GC stress C bug for arm (#15269)

Fix GC stress C bug for arm (#15269)

* Fix GC stress C bug for arm

The OnGcCoverageInterrupt function was not removing the "thumb" bit from
the address before putting back the original instruction that was
previously replaced by an invalid instruction that invokes the
OnGcCoverageInterrupt. That resulted in the original instruction being
restored to incorrect location (to the original address + 1) and later
to the GC stress failure.

* Reflect PR feedback