all: remove darwin/arm (32-bit)

[andybons]

Per #34751, we are removing darwin/arm (but keeping darwin/arm64).

[gopherbot]

Change https://golang.org/cl/222720 mentions this issue: dashboard: remove darwin-arm-mg912baios and darwin-arm64-mn4m2zdaios

[gopherbot]

Change https://golang.org/cl/227080 mentions this issue: cmd/dist: remove darwin/arm as a valid port

[gopherbot]

Change https://golang.org/cl/227198 mentions this issue: all: remove scattered remnants of darwin/arm

[gopherbot]

Change https://golang.org/cl/227197 mentions this issue: all: remove darwin/arm build-tags and files

[gopherbot]

Change https://golang.org/cl/226985 mentions this issue: cmd/dist: remove darwin/386, darwin/arm as valid ports

[gopherbot]

Change https://golang.org/cl/227181 mentions this issue: dashboard: update misc-compile--darwin description