msun: ctanh/ctanhf: Import fix from musl libc

This applies musl commit b02eed9c4841913d690a2d0029737d72615384fe by
Szabolcs Nagy and updates the tests accordingly. This also allows
removing an XFAIL from the test.

musl commit message:

complex: fix ctanh(+-0+i*nan) and ctanh(+-0+-i*inf)

These cases were incorrect in C11 as described by
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1886.htm

PR: 217528

Reviewed By: dim
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28578

(cherry picked from commit a7b42c4b7f7ad7bd1b22ab57ed9185bdcea6f0a2)