runtime/pprof: add a test for gccgo bug #29448

With gccgo, if a profiling signal arrives in certain time during
traceback, it may crash or hang. The fix is CL 156037 and
CL 156038.  This CL adds a test.

Updates #29448.

Change-Id: Idb36af176b4865b8fb31a85cad185ed4c07ade0c
Reviewed-on: https://go-review.googlesource.com/c/156018


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>