endian.h: Use the __bswap* versions

Make it possible to have all these macros work without bswap* being
defined. bswap* is part of the application namespace and applications
are free to redefine those functions.

Reviewed by: emaste,jhb,markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31964

(cherry picked from commit 44fb3c695f5e672ec00bdef0fa0f13793e6269a1)