[Bug target/82990] Add -mprefer-vzeroupper

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82990

--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> ---
We would like -mprefer-vzeroupper default to on and -mtune=knl should
override it.