Build gcc.debug using -Og flag

Since gcc 4.8 is required anyways, it might be nice to use its features.

Intro to feature (second bullet point):
https://gcc.gnu.org/gcc-4.8/changes.html

-g (line 328) is still needed:
http://stackoverflow.com/questions/12970596/gcc-4-8-does-og-imply-g
I just left it like this, since on lines 288, 323 and 328 a single item list is also split up into several lines... It might be useful if in the future additional flags are added and people don't have to "learn scons" to extend the list or start doing weird hacks like env.Append(CCFLAGS='-Og -Foo -Bar').