MFamd64: Explicitly initialize the mxcsr during npxinit().


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273989 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
MFamd64: Move extern declaration of _ucodesel and _udatasel to
<machine/md_var.h>


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273991 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f