Make sure that sh-mem-vec256 is only built on platforms that can
assemble the relevant instructions.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13502 a5019735-40e9-0310-863c-91ae7b9d1cf9
Minor tidying of the generalised V128/V256 shadow helper returns
that landed in r13500.  No functional change.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13503 a5019735-40e9-0310-863c-91ae7b9d1cf9