x86: Fix x86_64 'g' packet response to gdb from 32-bit mode.

The remote protocol can't handle flipping back and forth
between 32-bit and 64-bit regs. To compensate, pretend "as if"
on 64-bit cpu when in 32-bit mode.

Signed-off-by: Doug Evans <dje@google.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-Id: <001a113dca8274572005406e03c3@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>