Allow another prctl call.

A change was made so that pthread_create is calling
prctl(PR_PAC_RESET_KEYS, ...) on aarch64. It's possible that other
seccomp policies might need to change to allow this.

Test: CrasherTest.seccomp_backtrace passes on aarch64.
Change-Id: I9c4d1b3dca5f19a6285bf904bb942f1f52e42bd0