80-bit cube root

[Stephen L Moshier]

The file sysdeps/ieee754/ldbl-96/s_cbrtl.c does not seem to work.
It appears to have only double-precision accuracy.  For x86,
there is an assembly language version sysdeps/i386/fpu/s_cbrtl.S
that apparently is the one actually used.  That one is closer
to long double precision, though it shows about 2 ulps error.

Before looking further at the one in ldbl-96, I would like to
know, does anyone actually use it and has a problem been
noticed before?  There does not seem to be a cube root under
sysdeps/m68k/fpu, for example.