The parse_mem_bx function now returns a function to call to add an
action to the action list to handle the evaluation of the
displacement. This allows us to delay adding said action until
after we have emitted the actions for the instruction encodings
themselves.
Code like this should now work:
int x = 24
| st r1, x(sp)
This means that code like this can now be generated on s390x:
| ar r2, r3
| br r14
Still need to add support for immediates, memory, labels, other
instructions and so on.
Also sets the action list type to unsigned short (uint16_t) which
I think is the most appropriate type for s390x (x86 uses uint8_t
and other platforms use uint32_t).
