math/openblas: fix build on powerpc64 with TARGET=PPC970 and switch to PPC970

This patch fixes build for PPC970 target. Since most people that run FreeBSD on powerpc64 use PowerPC 970 CPU's, switch to it by default and add non-default option to optimize for POWER6 (optimizing for newer POWER doesn't work on BE).

Bump PORTREVISION because of package change.

PR: 238868
Approved by: phd_kimberlite@yahoo.co.jp (maintainer), linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D21010