runtime: enable race detector on openbsd/amd64

Now that this commit[1] has landed in LLVM the .syso file can be generated for
OpenBSD.

With the changes to src/runtime running the sample race[2] detects the data
race as expected.

Based on golang/go#39464 (https://go-review.googlesource.com/c/go/+/237057) from
Aaron Bieber <deftly@gmail.com>, however the race_openbsd_amd64.syso file has
been built on OpenBSD 6.4 and necessary changes added to race.bash.

[1] https://github.com/llvm/llvm-project/commit/fcf6ae2f070eba73074b6ec8d8281e54d29dbeeb
[2] https://golang.org/doc/articles/race_detector.html

Change-Id: Ic2479ccfa91d6b2cb4585346a11d813d96450f68
Reviewed-on: https://go-review.googlesource.com/c/go/+/275892


Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>