runtime/testdata/testprogcgo: refactor CrashTraceback

This moves the C part of the CrashTraceback test into its own file in
preparation for adding a test that transitions back into Go.

Change-Id: I9560dcfd80bf8a1d30809fd360f958f5261ebb01
Reviewed-on: https://go-review.googlesource.com/c/go/+/272130


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>