Refactor _LIBCPP_<LITTLE|BIG>_ENDIAN

Previously this macro used 0/1 to indicate if it was set.
This is unlike all other libc++ configuration macros which
use ifdef/ifndef.

This patch makes this macro consistent with everything else.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315995 91177308-0d34-0410-b5e6-96231b3b80d8