x86 nat: Add helper functions to save the XSAVE layout for the host.

x86 nat: Add helper functions to save the XSAVE layout for the host.

x86_xsave_length returns the total length of the XSAVE state area
standard format as queried from CPUID.

x86_fetch_xsave_layout uses CPUID to query the offsets of XSAVE
extended regions from the running host.  The total length of the XSAVE
state area can either be supplied by the caller if known (e.g. from
FreeBSD's PT_GETXSTATEINFO) or it can be queried from the running host
using x86_xsave_length.

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