crypto/internal/fips140/bigmod: vector implementation of addMulVVWx on s390x

crypto/internal/fips140/bigmod: vector implementation of addMulVVWx on s390x

addMulVVWx assembly routine is used to multiply bignum multiplicand with a 64-bit multiplier.
The new implementation for s390x architecture uses an algorithm based on vector instructions,
with a significant performance improvement.

Note: z13 is the minimum architecture for Go, which already has VX support.

The performance improvement is as below:

goos: linux
goarch: s390x
pkg: crypto/internal/fips140/bigmod
                  Orig.txt       Vector_Patch.txt
                   sec/op             sec/op          vs base
ModAdd          164.1n ± 0%   159.7n ± 0%      -2.7% (p=0.000 n=10)
ModSub          152.3n ± 1%   147.3n ± 0%      -3.25 (p=0.000 n=10)
MontgomeryRepr  4.806µ ± 3% 1.829µ ± 0%    -61.94% (p=0.000 n=10)
MontgomeryMul   4.812µ ± 5% 1.834µ ± 0%    -61.90% (p=0.000 n=10)
ModMul          9.646µ ± 3% 3.698µ ± 0%    -61.67% (p=0.000 n=10)
ExpBig          11.28m ± 0%   11.28m ± 0%      +0.04 (p=0.035 n=10)
Exp             12.284m ± 5%  5.004m ± 1%    -59.26  (p=0.000 n=10)
geomean         18.61µ        10.74µ         -42.2

Change-Id: I679944c9dac9f43f1626b018f72efa6da0d2442d
Cq-Include-Trybots: luci.golang.try:gotip-linux-s390x
Reviewed-on: https://go-review.googlesource.com/c/go/+/716480
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Vishwanatha HD <vishwanatha.hd@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Srinivas Pokala <Pokala.Srinivas@ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>