math: cosmetic cleanup (use explicit union instead of fshape and dshape)

math: fix nextafter and nexttoward on maxdbl and maxflt input

old code (return x+x;) returns correct value and raises correct
flags only if the result is stored as double (or float)

add FORCE_EVAL macro to evaluate float expr for their side effect
updated nextafter* to use FORCE_EVAL, it can be used in many other
places in the math code to improve readability.

fix unused variable warnings in new nextafter/nexttoward code

apparently initializing a variable is not "using" it but assigning to
it is "using" it. i don't really like this fix, but it's better than
trying to make a bigger cleanup just before a release, and it should
work fine (tested against nsz's math tests).

math: nextafter and nexttoward cleanup

make nexttoward, nexttowardf independent of long double representation.
fix nextafterl: it did not raise underflow flag when the result was 0.

first commit of the new libm!

thanks to the hard work of Szabolcs Nagy (nsz), identifying the best
(from correctness and license standpoint) implementations from freebsd
and openbsd and cleaning them up! musl should now fully support c99
float and long double math functions, and has near-complete complex
math support. tgmath should also work (fully on gcc-compatible
compilers, and mostly on any c99 compiler).

based largely on commit 0376d44a890fea261506f1fc63833e7a686dca19 from
nsz's libm git repo, with some additions (dummy versions of a few
missing long double complex functions, etc.) by me.

various cleanups still need to be made, including re-adding (if
they're correct) some asm functions that were dropped.