Make the host endianness check an integer constant expression.

I will remove the isBigEndianHost function once I update clang.

The ifdef logic is designed to
* not use configure/cmake to avoid breaking -arch i686 -arch ppc.
* default to little endian
* be as small as possible

It looks like sys/endian.h is the preferred header on most modern BSD systems,
but it is better to change this in a followup patch as machine/endian.h is
available on FreeBSD, OpenBSD, NetBSD and OS X.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179527 91177308-0d34-0410-b5e6-96231b3b80d8