SDM rev. 50 defines the use of the next 8 bytes in the xstate header.
It is the compaction bitmask, with the highest bit defining if compact
format of the xsave area is used at all.

Adjust the definition of struct xstate_hdr, provide define for bit 63.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271206 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f