rts: Fix build when linked with gold
rts: Fix build when linked with gold

As reported in #12812, the runtime system fails to build when linked
with gold due to a missing dependency on libpthread.

Additionally, rts/package.conf.in uses the WORD_SIZE_IN_BITS macro
defined by MachDeps.h, which it does not #include. Fix this.

Test Plan: Validate with gold linker

Reviewers: hsyl20, austin, erikd, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2695

GHC Trac Issues: #12816

(cherry picked from commit 2cfbee896be349d16238c044475c7c15cfb9b3f2)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc