In the previous implementation, the `stlxr` instruction clobbered
the value that was supposed to be returned by the the `xchg`
function.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>

Test Plan: build on aarch64

Reviewers: austin, bgamari, rwbarton

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D932

(cherry picked from commit f6ca6959)