Fix symlink install hook

During the install, symlinks are added from libunwind-<arch> to
libunwind-generic.  However, on platforms that don't support
symlinking (such as Windows), the $(LN_S) macro is defined
as 'cp -p' instead.  This works fine, except that since the
target of the symlink is a relative path, the copy will only
succeed if the current directory is the directory that contains
the file.

The solution to this problem suggested in the Autotools manual
(see http://www.gnu.org/software/automake/manual/automake.html#Extending)
is to simply cd into the correct directory first.  This patch
makes that change for the symlinks that are being made during install.