There have been a number of updates on the builders as well as in
the Go repo. This CL updates racebuild accordingly.

Add a Skip boolean to indicate platforms that are skipped by
default. This includes openbsd, for which the upstream TSAN
support is removed (#52090), and darwin/amd64v3, for which
currently the main Go repo doesn't use.

Specify SubArch for all AMD64 platforms. So the naming and file
path match what the main Go repo uses.

openbsd-amd64-70 builder is gone. Change to openbsd-amd64-72.

LLVM repo is very large. A git checkout takes a lot of disk space.
Some builders, however, don't have much disk space. Downloading a
zip file for the source code at the particular LLVM commit, and
extracting only compiler-rt gets all what we need, with a smaller
disk space usage.

Gomote gettar no longer works for retrieving a single file
(#64195). Work around it by putting the syso file in a directory
and getting that directory.

Also, gomote gettar no longer produces the tarball to stdout. It
writes a file instead. Change the download logic accordingly.

For golang/go#64195.

Change-Id: I49724d342a9278c977eb8d3fc1b9b32ea10e0b9a
Reviewed-on: https://go-review.googlesource.com/c/build/+/543015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>