[PATCH] Clarify XSAVE features, and always use XSAVE
1) Introduce npx_xcr0_mask, to keep track of which XCR0 features are enabled
2) Always use XSAVE regardless of whether AVX is supported or not. XSAVE may be
supported without AVX, on certain CPUs and also under hypervisors that expose
XSAVEOPT without AVX.