runtime: consider core PID in gdb test

runtime: consider core PID in gdb test

Add the PID to the core file name if the current system uses it
when generating core files.

Fixes #61487

Change-Id: I3b53a6c850c754795c8022921160f03c588d4c91
Reviewed-on: https://go-review.googlesource.com/c/go/+/511659
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>