Prompted by Andrew's report [1] I thought it may help if we accelerated two
of the patches I have pending as part of the (so far unposted) APX series.
We previously discussed latching some control register state once early, so
let's start with doing so for XCR0.

[1] https://lists.xen.org/archives/html/xen-devel/2026-08/msg01006.html

1: latch XCR0 early during decode
2: use latched XCR0 in x86emul_get_fpu()

Jan