fix microblaze atomic store

fix microblaze atomic store

as far as I can tell, microblaze is strongly ordered, but this does
not seem to be well-documented and the assumption may need revisiting.
even with strong ordering, however, a volatile C assignment is not
sufficient to implement atomic store, since it does not preclude
reordering by the compiler with respect to non-volatile stores and
loads.

simply flanking a C store with empty volatile asm blocks with memory
clobbers would achieve the desired result, but is likely to result in
worse code generation, since the address and value for the store may
need to be spilled. actually writing the store in asm, so that there's
only one asm block, should give optimal code generation while
satisfying the requirement for having a compiler barrier.