CI: add GHA build+test multi-platform workflow

[cce]

Runs C and Go tests on 5 platforms: Linux (x86, arm); Mac (x86, Apple Silicon); Windows.

Upgrades to Go 1.24 which fixes Windows cgo build error on the GHA runner, and also provides a builtin crypto/sha3 used in falcon_test.go (so now this library has no Go package dependencies)

[jannotti]

Something like this is very important for key derivation, to show that we get the same keys derived from the same seed on different platforms.

We probably don't have tests for that, because it has not been important before. But when we want to derive from 24 words, we need to show we get the same keys everywhere. We should have no problem, because we're emulating floating point.

[cce]

Something like this is very important for key derivation, to show that we get the same keys derived from the same seed on different platforms.

We'll have to add a big-endian target some day!

[nullun]

Nice! Thank you for adding this.