This fixes backend/setcc.c.

This reverts commit bf8ff592
and fixes opt/ifconv7.c.

We usually don't have capital letters in our function names.

- x86 extended float mantissa size includes the explicit one now
- fixed broken qnan/snan for x86 extended float
- simplification/tuning

This fixes opt/combo6.c.

Before this change the .so file would not mention libm as a dependency,
so users had to explicitely link -lm when linking -lfirm.

Add some documentation about this fact.

The tarval module has a maximally supported precision. Assert that no
modes with higher precision are created, otherwise the tarval module
will silently drop the higher bits on calculation. TODO: Change tarval
module to dynamically use smaller/bigger representations depending on
the mode.

Removed mode_LLs, mode_LLu, mode_Q for now as they are bigger than the
limit.