Rename the XCR0_* #defines to XFEATURE_* and add the new supervisor-state

Rename the XCR0_* #defines to XFEATURE_* and add the new supervisor-state
features: while all are appropriate for xsaves/xrstors, the
supervisor-state features aren't for xcr0 but rather for the new XSS_MSR,
making the current names kinda confusing.

Add #defines for masking bits for xcr0 vs XSS.

Add and report the new XSAVE_XFD xsave subfeature bit.

ok mlarkin@