src: simplify race.bash checking condition

src: simplify race.bash checking condition

By using "uname -s -m" to get the OS name and hardware name,
then using it to match supported platform in case command.

Change-Id: I5161a29c6f3fe34dcda9e7bd477fa3b772b9e041
Reviewed-on: https://go-review.googlesource.com/c/go/+/320250
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>