[MATH-EMU]: Fix underflow exception reporting.

The underflow exception cases were wrong.

This is one weird area of ieee1754 handling in that the underflow
behavior changes based upon whether underflow is enabled in the trap
enable mask of the FPU control register.  As a specific case the Sparc
V9 manual gives us the following description:

--------------------
If UFM = 0:     Underflow occurs if a nonzero result is tiny and a
                loss of accuracy occurs.  Tininess may be detected
                before or after rounding.  Loss of accuracy may be
                either a denormalization loss or an inexact result.

If UFM = 1:     Underflow occurs if a nonzero result is tiny.
                Tininess may be detected before or after rounding.
--------------------

What this amounts to in the packing case is if we go subnormal,
we set underflow if any of the following are true:

1) rounding sets inexact
2) we ended up rounding back up to normal (this is the case where
   we set the exponent to 1 ...