simplify as __builtin_inf has no arguments
adapt to new libfirm NaN handling and implement __builtin_nan payload
Avoid using dialect.strict setting in one instance

Instead we just emit a WARN_RETURN_TYPE warning in the parser and set
the warning to error if the -pedantic flag is given.

In the long term all dialect.strict usages should be changed to
something like this.
update libfirm revision to get big_endian fixes and verify changes