update Vector's AltiVec support

[Roland Haas]

Summit uses POWER9 processors, whose AltiVec SIMD engine accumulated bugs in Vectors. Pull request https://bitbucket.org/cactuscode/cactusutils/pull-requests/24/vectors-updates-for-altivec-and-knl/diff addresses these.

The pull request is from just before the Mayer release and has seen extensive discussion between @Erik Schnetter and myself as well as triggering creating a better test harness for Vectors (see #2380).

[Roland Haas]

Please review.

[Roland Haas]

Unless objected I will push this after the ET call on 2020-05-07.

[Roland Haas]

Applied as git hash 0aa7fd1 "Vectors: identify AVX512ER in vec_architecture string" of cactusutils