arm: Fix atomic long APIs to correct 'u_long' signedness

As defined in atomic(9) and implemented on other architectures, the
atomic(9) functions all act on unsigned pointers and types. Prior to this
revision, arm implemented some atomic(9) 'long' sized routines with correct
unsigned type, but others were incorrectly signed.

Reviewed by: tinderbox
Sponsored by: Dell EMC Isilon