Add script to setup codesigning on macOS

[Philippe Blain]

Hello, 

I’m interested in setting up a macOS build of GDB on conda-forge ([1], [2]) and it seems that in order to do it the codesigning procedure would have to be automated, i.e. entirely command-line driven. 
The LLDB project already has a script doing exactly that [3], and I was thinking of adding such a script to the conda-forge GDB feedstock. 
However, I thought it could be a good idea to add it directly to the GDB repository. Would a patch adding such a script be well received  ?

Thanks,
Philippe Blain.

[1] https://conda-forge.org
[2] https://github.com/conda-forge/gdb-feedstock/issues/7
[3] https://github.com/llvm/llvm-project/blob/master/lldb/scripts/macos-setup-codesign.sh