* arcsin and arccos: improve accuracy at small arguments by replacing sqr(1-x*x) with sqr((1-x)*(1+x)).
* arcosh: likewise, error near 1.0 decreases approx. from 80*eps to 4*eps.
* artahn: improve accuracy by replacing ln(x+1) with lnxp1(x).

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26091 3ad0048d-3df7-0310-abae-a5850022a9f2