[Bug libstdc++/86655] std::assoc_legendre should not constrain the value of m

[emsr at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86655

--- Comment #3 from emsr at gcc dot gnu.org ---
Sorry for missing this message.  You're right.
Ditto for sph_legendre.

Putting together a patch.