Add a "kernel" log function, based on e_log.c, which is useful for

Add a "kernel" log function, based on e_log.c, which is useful for
implementing accurate logarithms in different bases.  This is based
on an approach bde coded up years ago.

This function should always be inlined; it will be used in only a few
places, and rudimentary tests show a 40% performance improvement in
implementations of log2() and log10() on amd64.

The kernel takes a reduced argument x and returns the same polynomial
approximation as e_log.c, but omitting the low-order term. The low-order
term is much larger than the rest of the approximation, so the caller of
the kernel function can scale it to the appropriate base in extra precision
and obtain a much more accurate answer than by using log(x)/log(b).