Ben Gamari authored Jul 24, 2017 and Ben Gamari committed Jul 24, 2017

Since #847 we have used libthr due to reported hangs with FreeBSD's
KSE-based M:N pthread implementation. However, this was nearly 12 years
ago and today libpthread seems to work fine. Moreover, adding -lthr to
the linker flags break when used in conjunction with -r when gold is
used (since -l and -r are incompatible although BFD ld doesn't
complain).

Test Plan: Validate on FreeBSD

Reviewers: kgardas, austin

Subscribers: rwbarton, thomie

GHC Trac Issues: #847

Differential Revision: https://phabricator.haskell.org/D3773