pre-xl8 raw_mcontext for signals holds post-xl8 simd state

The dr_restore_state_info_t.raw_mcontext is supposed to hold the
pre-translation machine state in a dr_mcontext_t. However, the
signal-handling code only keeps a copy of the sigcontext_t prior to
translation. For x86, the simd state is shared between the pre- and
post- translation contexts. Thus if a client modifies simd state on
translation, the wrong values will be in raw_mcontext.