[libunwind] fix comment in compact_unwind_encoding.h

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@224656 91177308-0d34-0410-b5e6-96231b3b80d8
[libunwind] improve x86_64 comments in compact_unwind_encoding.h

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@224657 91177308-0d34-0410-b5e6-96231b3b80d8