Add libcxxabi's LICENSE.TXT to libunwind.

When libunwind was spinned off libcxxabi, most file were copied from
libcxxabi to libunwind.  However, libc++abi's toplevel LICENSE.TXT was
forgotten in the copying. It's considered a good practice to have the
license file at the root of the project, and making linunwind a separate
project was not supposed to change its licensing. Besides, several
header files refer to the LICENSE.TXT, so copy the one from libc++abi.

llvm-svn: 296358