math/big: add PCALIGN to addMulVVW asm on ppc64x

math/big: add PCALIGN to addMulVVW asm on ppc64x

Adding PCALIGN to addMulVVW assembler implementation
provides the following improvement on power10:

    AddMulVVW/1         3.36ns ± 0%    3.37ns ± 0%   +0.20%
    AddMulVVW/2         4.45ns ± 0%    4.44ns ± 0%   -0.25%
    AddMulVVW/3         5.44ns ± 0%    5.49ns ± 0%   +0.84%
    AddMulVVW/4         6.43ns ± 0%    6.34ns ± 0%   -1.33%
    AddMulVVW/5         7.87ns ± 0%    7.73ns ± 0%   -1.70%
    AddMulVVW/10        13.4ns ± 3%    12.4ns ± 7%   -7.07%
    AddMulVVW/100        112ns ± 0%     102ns ± 0%   -9.34%
    AddMulVVW/1000      1.09µs ± 0%    0.95µs ± 0%  -13.15%
    AddMulVVW/10000     10.9µs ± 0%     9.6µs ± 0%  -12.46%
    AddMulVVW/100000     109µs ± 0%      95µs ± 0%  -12.58%

Change-Id: Ic33d4f125c84d568f63e17cf99dc4df5ca9328d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/447236
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Archana Ravindar <ravindararchana@gmail.com>