builtins: avoid endian.h in favour of compiler builtins

Use the __BYTE_ORDER__ macro which has been available in GCC since 4.6 and is
available in clang as well.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@220626 91177308-0d34-0410-b5e6-96231b3b80d8