runtime: in traceback, print panic rather than runtime.gopanic

The gc toolchain does the same thing, in gentraceback in
runtime/traceback.go.

The test for this is TestPanicTraceback in runtime/crash_test.go.  We
don't yet run that test, but we will in a future change.

Change-Id: I72c6eef5cd8190b5ec804e6970fd7bc24f2fd3a4
Reviewed-on: https://go-review.googlesource.com/46397
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed