Simplify OS selection in Bazel by using @platforms.

Change-Id: I8711c2cc69db63d2d240c028ea016642eb77fb85
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59790
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed