Fix gc stress coverage handling of epilogs on X86 (#40432)

Fix gc stress coverage handling of epilogs on X86 (#40432)

Epilog checking relies on precise control of when instrumentation for the  first prolog
instruction is enabled or disabled. In particular, if a function has multiple epilogs, or
the first execution of the function terminates via an exception, and subsequent completions
do not, then the function may trigger a false stress fault if epilog checks are not disabled.

This fix makes it so that if the first instruction is hit during a GC coverage in a situation where
the epilog could be examined in the future, that the epilog verification is disabled.

Also an opportunistic fix for a probably unimportant race condition around checking the
callerThread during epilog processing.