Allow lib/msun/logarithm_test to pass on ld128 platforms

For some reason the ld128 log1pl() implementation is less accurate than
logl(), but does at least guarantee precision >= the ld80 implementation.
Mark log1p_accuracy_tests as XFAIL for ld128 and increase the log1p tolerance
to the ld80 equivalent in accuracy_tests to avoid losing test coverage for
the other functions.

PR: 253984
Reviewed By: ngie, dim
Differential Revision: https://reviews.freebsd.org/D29039