linux(4): In preparation for xsave refactor fxsave code on amd64

Due to fxsave area is os independent reimplement fxsave handmade code
using copying of a whole area.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D40443
MFC after: 2 weeks

(cherry picked from commit 920184ed6e43b19f4b71253b55fd3ba66a7f0bb2)