unwinder: conditionalise availability

__attribute__ (( unavailable )) is for Apple specific builds.  Create a macro
to conditionalise the usage of the macro.  This is to aid in porting the
unwinder to other platforms.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@192868 91177308-0d34-0410-b5e6-96231b3b80d8