maint: remove dependency on luabitop.

* specs/spec_helper.lua (bit): Fallback on posix.bit32 if there
is no native bit32 module.
* lib/posix/compat.lua, lib/posix/deprecated.lua: Use
posix.bit32 unconditionally.
* rockspec.conf (dependencies): Remove luabitop.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>