Matthias noticed that my backport of the Filesystem library removed
the libstdc++.so.6.0.21-gdb.py file, which was now getting installed
as libstdc++fs.a-gdb.py instead.

Fixed by changing the glob we use to find the candidate library files
that we use for the name of the installed gdb.py file.

Tested powerpc64le-linux, committed to trunk and gcc-5-branch.