Fix <endian.h> and <sys/endian.h>.

Previously we'd been relying on getting the machine-specific <endian.h>
instead of the top-level <endian.h>, and <sys/endian.h> was basically broken.
Now, with this patch and the previous patch we should have <endian.h>
and <sys/endian.h> behaving the same. This is basically how NetBSD's endian.h
works, and was probably how ours was originally intended to work.

Bug: http://code.google.com/p/android/issues/detail?id=39824
Change-Id: I733bd949014a648f2ea60d5ca02e758ae861a6ba