Correct debugserver to write xmm/ymm/zmm reg values

debugserver does not call thread_set_state when changing xmm/ymm/zmm
register values, so the register contents are never updated.  Fix
that.  Mark the shell tests which xfail'ed these tests on darwin systems
to xfail them when the system debugserver, they will pass when using
the in-tree debugserver.  When this makes it into the installed
system debugservers, we'll remove the xfails.

Differential Revision: https://reviews.llvm.org/D123269
rdar://91258333
rdar://31294382