x86: Add routines for querying XSAVE feature information

This patch adds several routines that track and expose information
about various XSAVE-related features. More specifically, it adds the
ability to check whether a given XFEATURE is supported and which XSAVE
extensions are supported. Furthermore, it adds several routines for
calculating the size and offsets within a save area given a XSAVE
feature bitmap.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D47394