cmd/dist: build exec wrappers during bootstrap

The androidtest.bash script encodes the additional steps to build
Go and run tests on Android. In order to add sharded builders and
trybots, Android needs to fit into the usual make.bash + cmd/dist test
pattern.

This change moves building the exec wrapper into cmd/dist bootstrap.

Do the same for iOS while we're here.

Updates #23824

Change-Id: I58a1b0679c3a6c92fdc7fff464b469641f1fee74
Reviewed-on: https://go-review.googlesource.com/c/163618


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>