runtime: skip TestGdbPanic on Windows

runtime: skip TestGdbPanic on Windows

TestGdbPanic expects crash() to raise a SIGABRT signal interceptable
by gdb, but Windows doesn't have signals.

Windows builders haven't caught this failing test because they still
don't have gdb installed (tracked in #22021).

Change-Id: I7c7f2523a54d61aea0a9821c4db7c79e58a7217c
Reviewed-on: https://go-review.googlesource.com/c/go/+/473116
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>