runtime: normalize line endings in TestGdbPython

runtime: normalize line endings in TestGdbPython

TestGdbPython fails on some Windows setups because the line endings in
the output of the gdb command can potentially be CRLF, but the test
expects LF.

This CL changes the test to normalize the line endings.

Change-Id: Iada9c37dc98249ac3c5f1edca4a657c51d4382a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/532816
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
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