runtime: save R15 before checking AVX state

When in dynlink mode, reading a global can clobber R15.
Just to be safe, save R15 before checking the AVX state to see
if we need to VZEROUPPER or not.

This could cause a problem in buildmodes that aren't supported yet.

Change-Id: I8fda62d3fbe808584774fa5e8d9810a4612a84e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/288452


Trust: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>