gdbserver: Use x86_xstate_layout to parse the XSAVE extended state area.

Replace the extended state area fields of i387_xsave with methods which
return an offset into the XSAVE buffer.

The two changed functions are called within all tests which runs
gdbserver.

Signed-off-by: Aleksandar Paunovic <aleksandar.paunovic@intel.com>
Co-authored-by: John Baldwin <jhb@FreeBSD.org>
Approved-By: Simon Marchi <simon.marchi@efficios.com>