fix(gdb): use POSIX-compatible find expression

We here use the technique for "-mindepth 1 -maxdepth 1" explained in

https://unix.stackexchange.com/a/330372/121088

We first suffix "/." to each directory name, and skip the top level
directory by "-name . -o".  Then we skip the level-1 directories and
their contents with "-type -d -prune -o".  Finally we check the
permission and print the files.

completions/gdb

@@ -25,13 +25,13 @@ _comp_cmd_gdb()
             # functions and aliases. Thus we need to retrieve the program
             # names manually.
             local path_array
-            _comp_compgen -Rv path_array split -F : -X '' -- "$PATH"
+            _comp_compgen -Rv path_array split -F : -X '' -S /. -- "$PATH"
             _comp_compgen_split -o plusdirs -- "$(
                 # Note: ${v+"$@"} does not work with empty IFS in bash < 4.4
                 IFS=$' \t\n'
-                find ${path_array[@]+"${path_array[@]}"} . -mindepth 1 \
-                    -maxdepth 1 -not -type d -executable -printf '%f\n' \
-                    2>/dev/null
+                find ${path_array[@]+"${path_array[@]}"} . -name . -o \
+                    -type d -prune -o -perm -u+x -print 2>/dev/null |
+                    command sed 's|^.*/||'
             )"
         fi
     elif ((cword == 2)); then