adapt to libfirm fp_model changes
Switch off strict-aliasing by default (for -O2 and above).

libFirm's type-based alias analysis is buggy: When a CopyB is lowered to
Loads and Stores, these have mode Iu instead of the actual modes of the
struct's members. The type-based alias analysis then computes wrong alias
relations.

This fixes the test-case "opt/fehler199.c".