[compiler-rt][msan] Guard shmat interceptor w SANITIZER_INTERCEPT_SHMCTL (#189198)

The shmat interceptor calls REAL(shmctl), but shmctl is not intercepted
on all targets (e.g. 32-bit Linux with musl). Guard shmat behind
SANITIZER_INTERCEPT_SHMCTL and use a MSAN_MAYBE_INTERCEPT pattern
consistent with other conditional interceptors.

GitOrigin-RevId: a8cdc5a483efc90e4699df25027ae4916115dfed