Implement load/store via simple LDR/STR for ARMv6+ (msftc)

Implement load/store via simple LDR/STR for ARMv6+ (msftc)

* src/atomic_ops/sysdeps/msftc/arm.h: Include
all_aligned_atomic_load_store.h instead of all_atomic_load_store.h
(since unaligned accesses are not guaranteed to be atomic).
* src/atomic_ops/sysdeps/msftc/arm.h (AO_load): Remove; include
all_aligned_atomic_load_store.h unconditionally instead (thus
implementing AO[_char/short]_load/store via simple LDR/STR; add
comment about Windows interrupt handlers.
* src/atomic_ops/sysdeps/msftc/arm.h: Add TODO item to implement
AO_test_and_set_full (for pre-ARMv6).