Use the POSIX timers API rather than itimer on FreeBSD (#49072)
POSIX.1-2008 marks `getitimer`/`setitimer` obsolete in favor of
`timer_gettime`/`timer_settime`. Additionally, POSIX.1-2017 marks
`SIGPROF` obsolete. Thus we can make simply have FreeBSD use the same
signals code paths as Linux, which already uses the timer API and uses
`SIGUSR1` rather than `SIGPROF`.
The code conditional on the `HAVE_ITIMER` flag, as well as the flag
itself, have been removed since they're no longer used.
