The code:

    uint64_t c = __sync_sub_and_fetch((uint64_t*)addr, 1);

was causing GCC to emit atomic instructions for 64 bit values which
are not available on PowerPC. However, since PowerPC only has a 32
bit address space, use of a 64 bit value is superflous.

Switching the type from `uint64_t` to `uintptr_t` should simply do
the correct thing on all 32 and 64 bit architectures.

Reviewers: austin, bgamari, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1399

GHC Trac Issues: #11036