acos.s fix: use the formula acos(x) = atan2(sqrt(1-x),sqrt(1+x))

[musl] / src / math / __signbitl.c

#include "libm.h"

// FIXME: should be a macro
#if (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
int __signbitl(long double x)
{
        union ldshape u = {x};

        return u.bits.sign;
}
#endif