libc++abi: silence some warnings

Cleans up the -Wundef warning caused by the use of the __LITTLE_ENDIAN__ macro.
Instead use `__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__`.  `__BYTE_ORDER__` is
defined by GCC since 4.2 and by clang.  This avoids the undef case where a macro
may be undefined.  This has previously caught real errors in libunwind.

llvm-svn: 235828