cmd/racebuild: update some builders
cmd/racebuild: update some builders

openbsd-amd64-race builder doesn't exist. Use openbsd-amd64-70
instead.

darwin-amd64-10_12 (i.e. macOS 10.12) is not longer supported.
Update to darwin-amd64-12_0.

Use linux-arm64-aws builder which is faster and more reliable.

Change-Id: Ic560e2d00f2c7a4e212adb2456605c51943395b6
Reviewed-on: https://go-review.googlesource.com/c/build/+/397474
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build