Enable eager FPU context switch by default on amd64.

Enable eager FPU context switch by default on amd64.

With compilers making increasing use of vector instructions the
performance benefit of lazily switching FPU state is no longer a
desirable tradeoff.  Linux switched to eager FPU context switch some
time ago, and the idea was floated on the FreeBSD-current mailing list
some years ago[1].

Enable eager FPU context switch by default on amd64, with a tunable/sysctl
available to turn it back off.

[1] https://lists.freebsd.org/pipermail/freebsd-current/2015-March/055198.html

Reviewed by:	jhb
Tested by:	pho
Sponsored by:	The FreeBSD Foundation