crypto/internal/bigmod: apply wasm-specific implementation for only sized addMulVVW

crypto/internal/bigmod: apply wasm-specific implementation for only sized addMulVVW

Restore generic addMulVVW for wasm (and therefore for all
architectures). Apply wasm-specific implementation for only the
explicitly sized functions (addMulVVW1024 etc.).

Also, for the sized functions, use unsafe pointer calculations
directly, without converting them back to slices. (This is what
the assembly code does on other architectures.) This results in a
bit more speedup for crypto/rsa benchmarks on Wasm:

pkg: crypto/rsa
                     │   old.txt   │              new.txt                │
                     │   sec/op    │   sec/op     vs base                │
DecryptPKCS1v15/2048   4.906m ± 0%   4.221m ± 1%  -13.96% (p=0.000 n=25)
DecryptPKCS1v15/3072   15.18m ± 0%   13.57m ± 0%  -10.64% (p=0.000 n=25)
DecryptPKCS1v15/4096   35.49m ± 0%   32.64m ± 1%   -8.04% (p=0.000 n=25)
EncryptPKCS1v15/2048   177.1µ ± 0%   162.3µ ± 0%   -8.35% (p=0.000 n=25)
DecryptOAEP/2048       4.900m ± 1%   4.233m ± 0%  -13.61% (p=0.000 n=25)
EncryptOAEP/2048       181.8µ ± 0%   166.8µ ± 0%   -8.24% (p=0.000 n=25)
SignPKCS1v15/2048      5.026m ± 1%   4.341m ± 0%  -13.63% (p=0.000 n=25)
VerifyPKCS1v15/2048    177.2µ ± 0%   161.3µ ± 1%   -8.97% (p=0.000 n=25)
SignPSS/2048           5.020m ± 0%   4.344m ± 1%  -13.47% (p=0.000 n=25)
VerifyPSS/2048         182.2µ ± 1%   166.6µ ± 0%   -8.52% (p=0.000 n=25)
geomean                1.791m        1.598m       -10.78%

Change-Id: I89775c46a0bbe29380889047ba393c6cfc093ff1
Reviewed-on: https://go-review.googlesource.com/c/go/+/628255
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>