Enable bssl (md5sum, sha256sum, etc.) on Windows.

We deal with the difference between binary and text modes on Windows by
doing all I/O in binary mode (including, in particular,
stdin/stdout/stderr) and by treating text mode as equivalent to binary
mode (i.e. we use Unix line ending semantics).

Change-Id: I76a46d8d02cd7efe1931c8272d8f2c311aef3acb
Reviewed-on: https://boringssl-review.googlesource.com/3070
Reviewed-by: Adam Langley <agl@google.com>