math: add dummy implementations of 128 bit long double functions

This is in preparation for the aarch64 port only to have the long
double math symbols available on ld128 platforms. The implementations
should be fixed up later once we have proper tests for these functions.

Added bigendian handling for ld128 bit manipulations too.

provide CMPLX macros in implementation-internal libm.h

this avoids assuming the presence of C11 macro definitions in the
public complex.h, which need changes potentially incompatible with the
way these macros are being used internally.

math: remove STRICT_ASSIGN macro

gcc did not always drop excess precision according to c99 at assignments
before version 4.5 even if -std=c99 was requested which caused badly
broken mathematical functions on i386 when FLT_EVAL_METHOD!=0

but STRICT_ASSIGN was not used consistently and it is worked around for
old compilers with -ffloat-store so it is no longer needed

the new convention is to get the compiler respect c99 semantics and when
excess precision is not harmful use float_t or double_t or to specialize
code using FLT_EVAL_METHOD

math: remove libc.h include from libm.h

libc.h is only for weak_alias so include it directly where it is used

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

math: remove *_WORD64 macros from libm.h

only fma used these macros and the explicit union is clearer

math: remove old longdbl.h

long double cleanup, initial commit

new ldshape union, ld128 support is kept, code that used the old
ldshape union was rewritten (IEEEl2bits union of freebsd libm is
not touched yet)

ld80 __fpclassifyl no longer tries to handle invalid representation

make CMPLX macros available in complex.h in non-c11 mode as well

math: turn off the STRICT_ASSIGN workaround by default

the volatile hack in STRICT_ASSIGN is only needed if
assignment is not respected and excess precision is kept.
gcc -fexcess-precision=standard and -ffloat-store both
respect assignment and musl use these flags by default.
i kept the macro for now so the workaround may be used
for bad compilers in the future.

complex: add C11 CMPLX macros and replace cpack with them

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.

add creal/cimag macros in complex.h (and use them in the functions defs)

don't inline __rem_pio2l so the code size is smaller

fix loads of missing const in new libm, and some global vars (?!) in powl

fix namespace issues for lgamma, etc.

standard functions cannot depend on nonstandard symbols

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.