gdbserver: Refactor the legacy region within the xsave struct

Legacy fields of the XSAVE area are already defined within fx_save
struct.  Use class inheritance to remove code duplication.

The two changed functions are called within all tests which run
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>