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
Merge branch 'master' into powerpc-darwin8
Move local Db type out to namespace scope.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@193169 91177308-0d34-0410-b5e6-96231b3b80d8
Rename LIBUNWIND_AVAIL to LIBUNWIND_UNAVAIL so as to not conflict with other (different) LIBUNWIND_AVAIL define

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@193839 91177308-0d34-0410-b5e6-96231b3b80d8
Swap contents of UnwindRegistersRestore.s and UnwindRegistersSave.s

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@193840 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' into powerpc-darwin8