runtime: traceback from outermost libc call

If we're in a libc call and get a trap, don't try to traceback the libc call.
Start from the state we had at entry to libc.

If there are multiple libc calls outstanding, remember the outermost one.

Fixes #26393

Change-Id: Icfe8794b95bf3bfd1a0679b456dcde2481dcabf3
Reviewed-on: https://go-review.googlesource.com/124195


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>