Enable USE_PTHREAD_FOR_ITIMER also on FreeBSD
If using a pthread instead of a timer signal is more reliable, and has no known drawbacks, then FreeBSD is also capable of supporting this mode of operation (tested on FreeBSD 12 with GHC 8.8.1, but no reason why it would not also work on FreeBSD 11 or GHC 8.6). If there are compelling drawbacks, this can be rejected, it is not a bugfix.
Proposed by Kevin Zhang in: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241849
If adopted, ideally this can be backported to 8.10.1, 8.8.2 (and perhaps even 8.6.6, if another release of 8.6 is expected).