[thevilledev]

semacreate allocates an unnamed POSIX semaphore on the C heap and
initializes it with sem_init, but mdestroy never released it. On AIX
sem_init fails with ENOSPC once SEM_NSEMS_MAX semaphores exist (32768
on AIX 7.3), so a program that keeps retiring Ms (for example a
goroutine that parks once and then returns while locked to its thread)
fails in semacreate with "fatal error: sem_init" after about 32000
retired threads. The malloc'd storage leaked as well.

Call sem_destroy and free from mdestroy, which runs after the exiting M
stops taking locks.

Add an AIX test that retires 40000 locked Ms that have each parked
once. It is in testprogcgo because without cgo, exited threads do not
exit on AIX until CL 849968 lands. Without the fix, the test fails with
"fatal error: sem_init".

Fixes #82305

[gopherbot]

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