cmd/racebuild: add linux/arm64
cmd/racebuild: add linux/arm64

Change-Id: Ieaac19bca46410a2d4f9648278a6307236d69e1e
Reviewed-on: https://go-review.googlesource.com/c/build/+/198397
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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