crypto: use purego tag consistently

[FiloSottile]

We've been inconsistent in using the purego tag in the standard library because it was not clear what would be its use aside from letting me benchmark assembly vs native code. I noticed on the Gophers Slack that tagging purego consistently would probably make TinyGo's life easier when trying to compile crypto packages, and presumably any other Go compiler. https://tinygo.org/docs/reference/lang-support/stdlib/#crypto

It's easy enough to do, so why not. Might also add a purego compilation test of crypto/... to longtest.

/cc @dgryski

[dgryski]

The hash/crc32 package also needs purego tags.

[cuonglm]

FYI, the purego proposal is accepted #23172

I also had https://go-review.googlesource.com/c/go/+/468795, which implements a purego version of hash/maphash. There, you can add the test with purego tag in cmd/dist.