misc/ios: add go_darwin_arm_exec script

This script is getting very close to complete, and is complex enough
that I'd like to get what's there so far reviewed. With it the builder
is left failing on eight packages. Two of those involve correcting
GOROOT which may need modifications to this script, the others are
either a unix sockets bug I have to hunt down or are caused by lldb
getting stuck on SIGSEGV, a TODO.

Change-Id: I5ff933800167b6764b51ad195da7dcda61d59ff8
Reviewed-on: https://go-review.googlesource.com/6404Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>