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

All of the tables describing the offsets of individual registers for
XSAVE state components now hold relative offsets rather than absolute
offsets.  Some tables (those for MPX registers and ZMMH registers) had
to be split into separate tables as they held entries that spanned
multiple state components.

Approved-By: Simon Marchi <simon.marchi@efficios.com>