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).
Proposed by Kevin Zhang in:
(cherry picked from commit 7385674cd386e8e5df189eb991040d85b07414f5)