[musl] casinh function accuracy problem

[mg1633068 <songyue@smail.nju.edu.cn>]

Hi everyone,
I'm writing testcases for complex math function. Considering the following simple code

#include <stdio.h>
#include <complex.h>
int main(int argc, char *argv[])
{
    double complex d = 3.0+6.6*I;
    double complex ret = casinh(d);
    printf("casinh(3.0+6.6*I)=%.15f+%.15f*I\n", creal(ret), cimag(ret));

    return 0;
}

With musl libc, the result is:
    casinh(3.0+6.6*I)=2.671002221994648+1.140551372972568*I
but with glibc, the result is:
    casinh(3.0+6.6*I)=2.671002221994652+1.140551372972565*I

We can see that musl is less accurate. I'm trying to solve this problem.
With little knowledge of numerical computing, any comment is appreciated!

Thank you in advance!

BR,
Song Yue