App Store policy requires programs do not reference the exc_server
symbol. (Some public forum threads show that Unity ran into this
several years ago and it is a hard policy rule.) While some research
suggests that I could write my own version of exc_server, the
expedient course is to disable the exception handler by default.

Go programs only need it when running under lldb, which is primarily
used by tests. So enable the exception handler in cmd/dist when we
are running the tests.

Fixes #10646

Change-Id: I853905254894b5367edb8abd381d45585a78ee8b
Reviewed-on: https://go-review.googlesource.com/9549Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>