runtime: deflake CGO traceback tests

The CGO traceback function is called whenever CGO code is executing and
a signal is received. This occurs much more frequently now SIGURG
is used for preemption.

Disable signal preemption to significantly increase the likelihood that
a signal results in a profile sample during the test.

Updates #37201

Change-Id: Icb1a33ab0754d1a74882a4ee265b4026abe30bdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/219417


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>