size_report: Use "Universal newlines" to fix ram_report on Windows

size_report was assuming that the GNU Binutils programs were
generating files with the line ending '\n'. But on native Windows
binutils will generate files with the line ending \r\n.

This patches size_report to use so-called "Universal newlines"[0], so
that size_report can deal with any kind of newline on any kind of
platform.

[0] https://www.python.org/dev/peps/pep-0278/



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>