amd64: do not enable PKRU if user disabled saving PKRU register in xsave mask

[kib]

Summary

This is done by reverting CR4_PKE bit, because we perform %CR4
initialization in initializecpu(), and the function is called before
xsave_mask is read.  To not redo the whole early initialization
sequence for the corner case, this should be good enough.

Reported by:    jhb

Also

x86: add more definitions for XCR0 bits

This covers all currently defined bits, adding PKRU and TILE.

[jhb]

I think for bhyve I might need to mask PKU as well, but I can deal with that part. bhyve also has to prevent guests from setting the CR4 bit.