cutils: add OS-independent endian.h

cutils/endian.h provides the helpers defined in endian(3), either by
pulling in the OS's built-in endian.h (where available) or recreating
them using GCC builtins.

Change-Id: Ic8965f67e1efdc03f884dbe6b7fe0276f840e4fc
Signed-off-by: Greg Hackmann <ghackmann@google.com>