Introduce a `_LIBCPP_HAS_BITSCAN64` macro to specify if the 64-bit variant of the bitscan family of APIs is available. This avoids duplicating the check in the support header. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290924 91177308-0d34-0410-b5e6-96231b3b80d8