Fix missing compiler barrier in AO_nop_full for uniprocessor ARM
(Apply commit b78058a from 'add-dbl-load-store' branch.)

* src/atomic_ops/sysdeps/armcc/arm_v6.h (AO_nop_full): Insert
AO_compiler_barrier for AO_UNIPROCESSOR case.
* src/atomic_ops/sysdeps/gcc/arm.h (AO_nop_full): Likewise.
* src/atomic_ops/sysdeps/msftc/arm.h (AO_nop_full): Likewise.
* src/atomic_ops/sysdeps/msftc/arm.h (AO_nop_full): Refine comment.

Conflicts:

	src/atomic_ops/sysdeps/msftc/arm.h
Fix return type of AO_char/short/int_load_read() in read_ordered.h

* src/atomic_ops/sysdeps/read_ordered.h (AO_char_load_read,
AO_short_load_read, AO_int_load_read): Change return type (and the
type of "result" local variable) from AO_t to the type matching the
name of the function (i.e., unsigned char/short/int, respectively).
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