Removing LazyFPU

[Maxime Villard <max%m00nbsd.net@localhost>]

I intend to enable EagerFPU by default, and gradually remove the LazyFPU code
from x86. The reasons for doing so are exactly the same as:

	https://lore.kernel.org/patchwork/patch/638220/
	https://lore.kernel.org/patchwork/patch/723722/

Basically, the LazyFPU code is pretty complex, and has performance issues. The
bugs in it are impossible to debug, like [1]. Since CVE-2018-3665, we already
have LazyFPU disabled by default on most Intel CPUs.

Removing LazyFPU will greatly simplify the FPU code; this is a prerequisite
for recent (but not so recent nowadays...) performance optimizations like
XSAVEOPT.

Note: in doing so I will enable EagerFPU on Xen PV, where it isn't enabled
currently. I don't have a setup to test the change, but it does nothing PV
cannot do, so it should work as-is.

Note: this won't make it to NetBSD 9. It may be pulled up later, however.

[1] https://gnats.netbsd.org/53017