x86: unify appropriate bits from dumpstack_32 and dumpstack_64
Impact: cleanup
As promised, now that dumpstack_32 and dumpstack_64 have so many bits
in common, we should merge the in-sync bits into a common file, to
prevent them from diverging again.
This patch removes bits which are common between dumpstack_32.c and
dumpstack_64.c and places them in a common dumpstack.c which is built
for both 32 and 64 bit arches.