Clean up _BYTE_ORDER definitions for better x86 portability.

We'd manually hacked _BYTE_ORDER into the arm and mips "_types.h" headers,
but not into the x86 one. Judging by upstream, _BYTE_ORDER should be in
the "endian.h" headers instead, so let's uniformly do that.

I've also ironed out some of the other differences between the different
architectures' header files too.

Bug: http://code.google.com/p/android/issues/detail?id=39824
Change-Id: I19d3af7ffd74e1c02b1b6886aec0f0d11f44ab8d