IAR: LDRT et al must be asm volatile

IAR: LDRT et al must be asm volatile

As these functions take volatile pointers, the API is promising that the
loads and stores will happen, so the assembler statements need volatile
qualifiers too.

If the functions took non-volatile pointers, or had a separate
non-volatile overload for C++, then the volatile could be omitted - the
instructions are normal loads and stores with no side-effects.

GCC and clang assembler already is "asm volatile", and armcc uses
intrinsics.

Change-Id: I4f5ce19ad732caa03ea9110501f0634175c85d4e