Lock the trampoline when attaching

When attaching to the trampoline, the isc__trampoline_max was access
unlocked.  This would not manifest under normal circumstances because we
initialize 65 trampolines by default and that's enough for most
commodity hardware, but there are ARM machines with 128+ cores where
this would be reported by ThreadSanitizer.

Add locking around the code in isc__trampoline_attach().  This also
requires the lock to leak on exit (along with memory that we already)
because a new thread might be attaching to the trampoline while we are
running the library destructor at the same time.
72 jobs for !6257 with ondrej-fix-trampoline-locking in 45 minutes and 39 seconds (queued for 7 seconds)
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