msun: remove fabs from Symbol.map, and adjust comment

We have s_fabs.c, but fabs(3) is already provided by libc due to
historical reasons, so it is not compiled into libm. When the linker
does not use --undefined-version, this leads to a complaint about the
symbol being nonexistent, so remove it from Symbol.map.

While here, adjust the comment about some functions being supplied by
libc: while it is true that all these are indeed in libc, libm still
includes its own versions of frexp(3), isnan(3), isnanf(3), and
isnanl(3).

Reported by: Steve Kargl <sgk@troutmask.apl.washington.edu>
MFC after: 3 days