proposal: cmd/compile: support GOARM64=v9.6

[nixiajun886-coder]

Proposal Details

GOARM64 currently supports architecture versions through v9.5.

I propose adding GOARM64=v9.6 and the arm64.v9.6 build tag. FEAT_CMPBR
is mandatory in Armv9.6-A, so it should be enabled automatically for
v9.6 rather than exposed as a separate ,cmpbr option.

This requires coordinated updates to build configuration, runtime
feature checks, documentation, build tags, and the MinimumRequirements
wiki.

Related prototype: https://go-review.googlesource.com/c/go/+/800041

I would like guidance on whether GOARM64=v9.6 support should land as a
separate foundational change before the CMPBR compiler changes.

[gabyhelp]

Related Issues

• all: add GOARM64=v8.1 and so on #60905 (closed)
• cmd/go: provide build tags for architecture environment variables #45454 (closed)
• proposal: cmd/compile: add GORISCV64=g #74540

Related Code Changes

• internal/buildcfg,runtime: add GOARM64 v9.6 CMPBR support

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

cc @golang/arm

[cherrymui]

Sounds reasonable to add the option. Are there v9.6 machines available now, or is this mostly for the future?

[nixiajun886-coder]

Armv9.6 is available as a specification, but there are no widely available commercial machines yet. For now, it is mostly aimed at future hardware.