Ben Gamari authored Feb 26, 2017 and Ben Gamari committed Feb 28, 2017

Some platforms have pthreads support available without linking against
libpthread (and indeed don't even offer a libpthread to link against).
One example of this is Android's bionic library. Teach the RTS about
this case.

Test Plan: Validate while cross-compiling targetting Android on aarch64

Reviewers: simonmar, austin, hvr, erikd, rwbarton

Subscribers: danharaj, thomie, erikd, snowleopard

Differential Revision: https://phabricator.haskell.org/D3149