[M68k][AsmParser] Support parsing register masks & fix printing them

Fixes PR51580.

Register masks will now be printed as 'movem.l (%sp), %a0-%a5/%d5'
for example and can now be parsed in the same format.

Previously the printed syntax was 'movem.l (%sp), %a0-%a5,%d', which
didn't match prior art and was too ambiguous to easily parse.

Differential Revision: https://reviews.llvm.org/D108597