cmd/dist,internal: add GOARM64 environment variable

cmd/dist,internal: add GOARM64 environment variable

Adds GOARM64 environment variable with accepted range of values "v8.{0-9}",
"v9.{0-5}" and optional ",lse" and ",crypto" suffixes.

Right now it doesn't affect anything, but can be used in the future to
selectively target specific versions of different ARM64 hardware.

For #60905

Change-Id: I6d530041b6931aa884e34f719f8ec41b1cb03ece
Reviewed-on: https://go-review.googlesource.com/c/go/+/559555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Shu-Chun Weng <scw@google.com>
Reviewed-by: Fannie Zhang <Fannie.Zhang@arm.com>