Preprocessor: add __BYTE_ORDER__ predefined macro

The __BYTE_ORDER__ predefined macro was added in GCC 4.6:
http://gcc.gnu.org/onlinedocs/gcc-4.6.0/cpp/Common-Predefined-Macros.html

It's used like the following:

 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 ...
 #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 ...
 #else
 #error insane architecture like the pdp-11
 #endif

There's a similar macro, __FLOAT_WORD_ORDER__, but it looks like it
mainly exist to accommodate fairly obscure architectures and ARM's
old FPA instructions, so it doesn't seem nearly as useful.

The tests are updated to check for the correct(at least, based on
clang's current output) value of the macro on each target. So now the
suite will catch bugs like the one fixed in r157626.

llvm-svn: 160879