Improved architecture auto detection.
Improved architecture auto detection.

Improvements:
- added support for -march=atom
- fixed some assignments of incompatible enum types
- simplified code

davidgiven/libfirm,killbug2004/libfirm,jonashaag/libfirm,killbug2004/libfirm,MatzeB/libfirm,libfirm/libfirm,davidgiven/libfirm,MatzeB/libfirm,davidgiven/libfirm,MatzeB/libfirm,jonashaag/libfirm,jonashaag/libfirm,libfirm/libfirm,8l/libfirm,MatzeB/libfirm,MatzeB/libfirm,jonashaag/libfirm,MatzeB/libfirm,killbug2004/libfirm,killbug2004/libfirm,libfirm/libfirm,8l/libfirm,8l/libfirm,jonashaag/libfirm,killbug2004/libfirm,MatzeB/libfirm,davidgiven/libfirm,davidgiven/libfirm,libfirm/libfirm,jonashaag/libfirm,8l/libfirm,8l/libfirm,jonashaag/libfirm,davidgiven/libfirm,davidgiven/libfirm,libfirm/libfirm,killbug2004/libfirm,killbug2004/libfirm,8l/libfirm,8l/libfirm