math/openblas: fix build on powerpc64

This port needs a patch (modified for kernel/power/*.S which differ) from https://github.com/xianyi/OpenBLAS/pull/2169 to build on powerpc64.

Since optimizing for PPC970 ends with an error, optimize for POWER6 on powerpc64.

PR: 238825
Approved by: phd_kimberlite@yahoo.co.jp (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20777