* use unsigned arithmetics on the representation
* store arg reduction quotient in unsigned (so n%2 would work like n&1)
* use different convention to pass the arg reduction bit to __tan
 (this argument used to be 1 for even and -1 for odd reduction
 which meant obscure bithacks, the new n&1 is cleaner)
* raise inexact and underflow flags correctly for small x
 (tanl(x) may still raise spurious underflow for small but normal x)
 (this exception raising code increases codesize a bit, similar fixes
 are needed in many other places, it may worth investigating at some
 point if the inexact and underflow flags are worth raising correctly
 as this is not strictly required by the standard)
* tanf manual reduction optimization is kept for now
* tanl code path is cleaned up to follow similar logic to tan and tanf

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.