build: arm64 builders don't support SHA-512 extensions

[FiloSottile]

The arm64 builders don't support the SHA-512 Armv8 extensions, so our crypto/sha512 assembly is untested, in violation of the Assembly Policy.

/cc @golang/security @golang/release

[gabyhelp]

Related Issues and Documentation

• build: amd64 builders don't support SHA extensions #69592

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[gopherbot]

Change https://go.dev/cl/614656 mentions this issue: crypto/sha256,crypto/sha512: test fallback implementations

[rolandshoemaker]

The new GCP C4A instances appear to support the SHA-512 instructions.

[FiloSottile]

I ran into this again for CL 667675, but I realized we have Apple M builders, which do have the Armv8.2 instructions, so we can use those. It's unfortunate we don't have a darwin/arm64 longtest, though.

[gopherbot]

Change https://go.dev/cl/667755 mentions this issue: crypto/internal/cryptotest: test Armv8.2 on darwin/arm64

[prattmic]

As part of #73397, I've turned up new darwin-arm64 testing infra, increasing our capacity from 5 to 30 machines. We were just discussing that a good next step would be to add darwin-arm64 longtest and race builders. Feel free to open an issue.