Fix ilogb(3) implementation. The results have to match FP_ILOGB0 and
FP_ILOGBNAN which isn't the case for the amd64 and i386 assembly versions.
Drop these in favour of C implementations. Als reimplement ilogbl(3)
by providing separate ld80 and ld128 implementations that replace the
existing implementation which may hit an infinite loop when built for
quad-precision long double.

ok patrick@, gkoehler@