runtime: use a 64kb system stack on arm

runtime: use a 64kb system stack on arm

I went looking for an arm system whose stacks are by default smaller
than 64KB. In fact the smallest common linux target I could find was
Android, which like iOS uses 1MB stacks.

Fixes #11873

Change-Id: Ieeb66ad095b3da18d47ba21360ea75152a4107c6
Reviewed-on: https://go-review.googlesource.com/14602
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Minux Ma <minux@golang.org>