cmd/compile: enable -d=checkptr when -race or -msan is specified

cmd/compile: enable -d=checkptr when -race or -msan is specified

It can still be manually disabled again using -d=checkptr=0.

It's also still disabled by default for GOOS=windows, because the
Windows standard library code has a lot of unsafe pointer conversions
that need updating.

Updates #34964.

Change-Id: Ie0b8b4fdf9761565e0dcb00d69997ad896ac233d
Reviewed-on: https://go-review.googlesource.com/c/go/+/201783
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>