Fix ARM atomics for 8- and 16-bit types

This has apparently never worked for any negative value. That's because
the compiler sign-extends the incoming expectedValue to fill the 32-bit
register, instead of zero-extending it, and it also expects any returned
values to also be sign-extended.

Task-number: QTBUG-37031
Change-Id: I836eddba7b1acc56bb0ac1d41de7001d06255b9b
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>