accuracy of long double functions

with Openlibm 0.7.5 I still see the following issues for long double functions:

* a somewhat large maximal known error for atanhl (85.4 ulps)
* huge error for lgammal (9.08e19 ulps)
* huge error for tgammal (#223)
* large error for powl (533 ulps) and powl does not work properly in multi-thread mode (#222)

For all other functions, the maximal known error is 5.67 ulps
(see https://members.loria.fr/PZimmermann/papers/accuracy.pdf).