fix misaligned pointer-like objects in arm atomics asm source file

fix misaligned pointer-like objects in arm atomics asm source file

this file's .data section was not aligned, and just happened to get
the correct alignment with past builds. it's likely that the move of
atomic.s from arch/arm/src to src/thread/arm caused the change in
alignment, which broke the atomic and thread-pointer access fragments
on actual armv5 hardware.