The Go builders (and standard development cycle) for programs on iOS
require running the programs under lldb. Unfortunately lldb intercepts
SIGSEGV and will not give it back.

https://llvm.org/bugs/show_bug.cgi?id=22868

We get around this by never letting lldb see the SIGSEGV. On darwin,
Unix signals are emulated on top of mach exceptions. The debugger
registers a task-level mach exception handler. We register a
thread-level exception handler which acts as a faux signal handler.
The thread-level handler gets precedence over the task-level handler,
so we can turn the exception EXC_BAD_ACCESS into a panic before lldb
can see it.

Fixes #10043

Change-Id: I64d7c310dfa7ecf60eb1e59f094966520d473335
Reviewed-on: https://go-review.googlesource.com/7072Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>