floatval=long double: fix fdiv_i_i neg, add fabsl

also fix ceil_n, floor_n for long double.
use ceill and floorl only with long double.

fix most long double tests,
just the embed API is still broken for long double:
t/src/embed/pmc.t                         (Wstat: 256 Tests: 9 Failed: 1)
  Failed test:  2
t/src/extend_vtable.t                     (Wstat: 3840 Tests: 134 Failed: 15)
  Failed tests:  7, 22, 46-47, 59, 72, 80-81, 89-90, 96-97
                103-104, 128

Missing: long variants for more float methods, like sin, cos, ...
Closes GH #1111