crossbuild.bash: set GOARM=7

From https://github.com/golang/go/wiki/GoArm :

  In cross compilation situations, it is recommended
  that you always set an appropriate GOARM value
  along with GOARCH.

The value seems to default to GOARM=5 if not set
during cross-compilation.

crossbuild.bash

@@ -6,8 +6,11 @@ B="go build -tags without_openssl"
 
 set -x
 
-GOOS=linux  GOARCH=arm   $B
+# See https://github.com/golang/go/wiki/GoArm
+GOOS=linux  GOARCH=arm   GOARM=7 $B
 GOOS=linux  GOARCH=arm64 $B
+
+# MacOS
 GOOS=darwin GOARCH=amd64 $B
 
 # The cross-built binary is not useful on the compile host.