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