libgo: Build math package with -ffp-contract=off on non-x86.

http://golang.org/issue/7074 shows that not using
-ffp-contract=off produces the wrong result for math.Log2(1)
on arm64.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@208505 138bc75d-0d04-0410-961f-82ee72b054a4