gcc47: Allow gcc47 to compile itself at WORLD_CCOPTLEVEL=g.

gcc47: Allow gcc47 to compile itself at WORLD_CCOPTLEVEL=g.

Override supplied -Og when gcc47 is bootstrapping itself.
-Og is very useful in some heavy debugging seasions so it is better to give
developers an easy workaround than fail for a single part of buildworld.

Only intended for WORLD_CCOPTLEVEL=g compilations to not require to explictly
disable the alternative compiler (NO_ALTCOMPILER=true).
Note that -Og is mostly -O1 with few optimizations disabled.