crypto: use purego tag consistently #58636
Description
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