cmd/compile: test delve instead of gdb in ssa/debug_test.go

This seems to deflake the test, and also allows testing
on macOS.

Fixes #31786.

Change-Id: I10bfba46dd4b8e64cb09fdd4dd9d175c1ce1f022
Reviewed-on: https://go-review.googlesource.com/c/go/+/176058


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>