Clean-up _fpmath and fake_long_doubles

- promoted IEEEld2bits to fpmath since most of the where the same for
diffrent archs
- removed _fpmath
- reinstated weak_references
- moved isfinite and isnormal to libc
- clean up fake_long_doubles
- clean up some useless ifdefs
- added missing nexttoward* tests

Bug: 14134235
Change-Id: I95639c4885653fe47fd7dc0570ee5bb3389bbc6b