math: remove comment about aliasing lgamma as gamma
It is probably not worth supporting gamma.
(it was already deprecated in 4.3BSD)

other side of the signgam namespace fix: use the internal name

fix namespace issues for lgamma, etc.

standard functions cannot depend on nonstandard symbols

in math.h make lgamma_r and non-double bessel _GNU_SOURCE only
long double and float bessel functions are no longer xsi extensions

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.