gdb: Support XSAVE layouts for the current host in the Linux x86 targets.

gdb: Support XSAVE layouts for the current host in the Linux x86 targets.

Note that this uses the CPUID instruction to determine the total size
of the XSAVE register set.  If there is a way to fetch the register set
size using ptrace that would probably be better.

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