amd64: panic on -CURRENT _at_r330539 for certain UEFI hosts

Some recent UEFI implementations have begun to leave the CPU with page
write protection enabled in CR0.

With r330539 which enables kernel page protections, interesting things
happen during boot (aka panic) when protection is already enabled,
including a write protection fault from an explicit .text fixup write
from xsave->xsaveopt by fpuinit().

I see this so far booting -CURRENT under virtual environments:

- QEMU with recent OVMF EDK2 builds: this is certainly due to UEFI
enabling paging and page protections.

- VMWare Fusion 10.1.x on Mac: no specific insight on what's going
inside the implementation, but CR0_WP is definitely left enabled before
the kernel is booted.

I have patched my kernel build to explicitly clear CR0_WP (e.g. in
initializecpu) prior to creating the page tables to get around this, but
someone might have a cleaner/better solution...

--peter