cmd/compile: enable -d=checkptr even on windows

CL 201783 enable -d=checkptr when -race or -msan is specified
everywhere but windows.

But, now that all unsafe pointer conversions in the standard
library are fixed, enable -d=checkptr even on windows.

Updates #34964
Updates #34972

Change-Id: Id912fa83b0d5b46c6f1c134c742fd94d2d185835
Reviewed-on: https://go-review.googlesource.com/c/go/+/227003


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>