runtime: improve Windows gdb tests

runtime: improve Windows gdb tests

This ensures that gdb tests run on Windows by ignoring any line ending.

Works with gdb 7.7, however with gdb 7.9 and 7.12 gets an error
error:

    internal-error: buildsym_init: Assertion `free_pendings == NULL'
    failed.

Updates #21380

Change-Id: I6a6e5b2a1b5efdca4dfce009fcb9c134c87497d6
Reviewed-on: https://go-review.googlesource.com/102419
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>