use (1-x)*(1+x) instead of (1-x*x) in asin.s
the later can be inaccurate with upward rounding when x is close to 1

the int part was wrong when -1 < x <= -0 (+0.0 instead of -0.0)
and the size and performace gain of the asm version was negligible

cleaner implementation with unions and unsigned arithmetic

modfl(+-inf) was wrong on ld80 because the explicit msb
was not taken into account during inf vs nan check

previously a division was accidentally turned into integer div
(w = -i/NXT;) instead of long double div (w = -i; w /= NXT;)

It is probably not worth supporting gamma.
(it was already deprecated in 4.3BSD)

(fldl instruction was used instead of flds and fldt)