Setup fallback 'debug-file-directory'

[tesuji]

Description

This PR adds fallback path /usr/lib/debug to debug-file-directory config.
Closes #1169.

Some gdbs installed outside /usr (like nix or in user homedir) will not search
for dbgsyms in /usr/lib/debug. However /us/lib/debug is where most distros
package debuginfo symbols to.

Before

> /run/workspace/bin/gdb -q -x .gef.py -ex 'show debug-file-directory' -ex 'quit'
The directory where separate debug symbols are searched for is "/nix/store/k51rfv38p65jbhw906vpl85yg1vd7xqn-gdb-15.2/lib/debug".

After

> /run/workspace/bin/gdb -q -x .gef.py -ex 'show debug-file-directory' -ex 'quit'
The directory where separate debug symbols are searched for is "/nix/store/k51rfv38p65jbhw906vpl85yg1vd7xqn-gdb-15.2/lib/debug:/usr/lib/debug".

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

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