cmd/dist: remove arch-specific timeout scale heuristics; handle these in x/build as needed

[dmitshur]

Discussion in #56968 seems to suggest we have agreement to get cmd/dist out of the practice of maintaining a hardcoded timeout scale for a few architectures. Instead, dist users can set it for their own machine (if at all needed) and we can factor out this configuration into x/build/dashboard for builders.

That is, we should remove these lines from cmd/dist:

switch goarch {
case "arm":
	t.timeoutScale = 2
case "mips", "mipsle", "mips64", "mips64le":
	t.timeoutScale = 4
}

And adjust x/build as needed to avoid breaking any builders that might rely on that.

Breaking this off into this issue for tracking purposes.

CC @aclements, @bcmills.

[gopherbot]

Change https://go.dev/cl/455518 mentions this issue: cmd/dist: remove hardcoded timeout scale for arm and mips{,le,64,64le}

[gopherbot]

Change https://go.dev/cl/455521 mentions this issue: dashboard: apply cmd/dist's timeout scale for arm, mips* architectures