Summary:
These primops were failing to compile on PowerPC (32 bit). There is also
currently no way to call into these primops from Haskell code. Currently,
the *only* way to call any of these C hs_atomic_* functions is via the
fetch*IntArray primops which are only defined for Int values and Int is
always the native word size.

When these functions can be called (and tested) from Haskell code, then
it will be worth while implementing them.

Test Plan:
    Compile and run on x86, x86_64, powerpc and arm:
    testsuite/tests/concurrent/should_run/AtomicPrimops.hs

Reviewers: tibbe, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D702

GHC Trac Issues: #9886