Restore buildworld from clang (adjust libm)

Restore buildworld from clang (adjust libm)

Functions like "isinff" come from GCC only, causes a missed reference when
the world is built with clang.  To solve, bring back FreeBSD versions of
round(|f|l).  An additional benefit is that these functions appear to be
improved over the OpenBSD versions (avoids unnecessary conversions).  The
FreeBSD roundl uses ENTERI/RETURNI macros so I left these off meaning DF
will still not able to raise exceptions on roundl.

Since isinff and friends are no longer used, clang can continue further with
building the world.