runtime: rename tracefpunwindoff GODEBUG

[nsrip-dd]

Go 1.21 added GODEBUG=tracefpunwindoff as a way to opt out of frame pointer unwinding when it was added for the execution tracer. Go 1.23 started using frame pointer unwinding in the block and mutex profilers, using the same GODEBUG to opt out of frame pointer unwinding. tracefpunwindoff no longer seems like an apt name since we don't just use it for the execution tracer. We may use frame pointer unwinding in other parts of the runtime in the future. We probably don't need more than one opt-out for all the different places it's used. In CL 718522, @mknyszek suggested either:

• Introduce a more general GODEBUG that aliases tracefpunwindoff, then drop tracefpunwindoff next release, or
• Rename it in Go 1.26 and announce it.

This issue tracks that decision. I suggest fpunwindoff for the new name.

See also #76163, which proposes a policy for removing GODEBUGs.

cc @golang/runtime

[gabyhelp]

Related Issues

• runtime: GODEBUG=profstackdepth=N not usable with //go:debug #71187
• runtime: enable `debugCheckBP` by default for language version 1.26 #75490

Related Code Changes

• runtime: document that tracefpunwindoff applies to some profilers

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

[mknyszek]

I like the proposed name. I have no further comments, we just need to decide.

[mknyszek]

CC @golang/runtime