Finalize work on the math library. It's time to do this monster
commit, and deal with problems (if any) in tree.

Note that this adds the following functions. Ports with hacks might
need adjustments.

nexttoward(3), fma(3), nexttowardf(3), fmaf(3), acoshl(3), asinhl(3),
atanhl(3), coshl(3), sinhl(3), tanhl(3), expl(3), expm1l(3), logl(3),
log10l(3), log1pl(3), log2l(3), modfl(3), cbrtl(3), hypotl(3),
powl(3), erfl(3), erfcl(3), lgammal(3), tgammal(3), ceill(3),
floorl(3), lrintl(3), llrintl(3), roundl(3), lroundl(3), llroundl(3),
truncl(3), fmodl(3), remainderl(3), remquol(3), nextafterl(3),
nexttowardl(3), fmal(3).

With this commit, our library implements all functionality required
by C99. Documentation bits will follow.