dashboard: add arm64 buildlet, update ppc64 buildlet definition
dashboard: add arm64 buildlet, update ppc64 buildlet definition

Change-Id: I25c6de0f97c3338f9d32f4125b2c7c68c3f3c13c
Reviewed-on: https://go-review.googlesource.com/21656
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

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