math: fix rare underflow issue in fma

the issue is described in commits 1e5eb73545ca6cfe8b918798835aaf6e07af5beb
and ffd8ac2dd50f99c3c83d7d9d845df9874ec3e7d5

math: fix remaining old long double code (erfl, fmal, lgammal, scalbnl)

in lgammal don't handle 1 and 2 specially, in fma use the new ldshape
union instead of ld80 one.

math: add fma TODO comments about the underflow issue

The underflow exception is not raised correctly in some
cornercases (see previous fma commit), added comments
with examples for fmaf, fmal and non-x86 fma.

In fmaf store the result before returning so it has the
correct precision when FLT_EVAL_METHOD!=0

math: use '#pragma STDC FENV_ACCESS ON' when fenv is accessed

use scalbn or *2.0 instead of ldexp, fix fmal

Some code assumed ldexp(x, 1) is faster than 2.0*x,
but ldexp is a wrapper around scalbn which uses
multiplications inside, so this optimization is
wrong.

This commit also fixes fmal which accidentally
used ldexp instead of ldexpl loosing precision.

There are various additional changes from the
work-in-progress const cleanups.

fmal bug fix: nan input should not raise exception

make fma and lrint functions build without full fenv support

this is necessary to support archs where fenv is incomplete or
unavailable (presently arm). fma, fmal, and the lrint family should
work perfectly fine with this change; fmaf is slightly broken with
respect to rounding as it depends on non-default rounding modes to do
its work.

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.