cmd/buildlet: build netbsd-arm version with GOARM=7

Otherwise, the resulting binary refuses to run on a multicore machine,
stating that the use of atomic synchronization instructions is needed.

Change-Id: Ie1a626e064a77c4fd80abbdc597ddbc62e3ffb95
Reviewed-on: https://go-review.googlesource.com/c/build/+/174141
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/buildlet/Makefile b/cmd/buildlet/Makefile
index 0c5660c..dd025cb 100644
--- a/cmd/buildlet/Makefile
+++ b/cmd/buildlet/Makefile
@@ -60,7 +60,7 @@
 
 buildlet.netbsd-arm: FORCE
 	go install golang.org/x/build/cmd/upload
-	upload --verbose --osarch=$@ --file=go:golang.org/x/build/cmd/buildlet --public --cacheable=false go-builder-data/$@
+	upload --verbose --extraenv=GOARM=7 --osarch=$@ --file=go:golang.org/x/build/cmd/buildlet --public --cacheable=false go-builder-data/$@
 
 buildlet.netbsd-386: FORCE
 	go install golang.org/x/build/cmd/upload