soft-fp: Fix used without set warning in _FP_MUL and _FP_DIV

soft-fp: Fix used without set warning in _FP_MUL and _FP_DIV

Seen in, e.g. ports/sysdeps/alpha/soft-fp/ots_mul.c.

The problem here is we have a switch on the class of X*Y, followed by
a switch on the class of R.  The exponent field of R really is not set
by the first switch for NaN outputs, because we know it's not going to
be used.  The compiler is not smart enough to see through this.

By pulling the exponent computation out of the NORMAL*NORMAL path, we
will not be pessimizing the most common case, while still providing an
arbitrary value to satisfy the compiler.