Fix rust-lang#111961 r=Mark-Simulacrum

Makes the Python pretty printer library source'able from within
GDB after spawn.

This makes `rust-gdb` not the required approach. It also provides the possibility
for GUI:s that wrap GDB, to debug Rust using the pretty printer library; as well
as other projects such as for instance Pernosco, which previously was not possible.

This won't introduce any new unexpected behaviors for users of `rust-gdb`

Author: theIDinside

src/etc/gdb_load_rust_pretty_printers.py

@@ -1,3 +1,14 @@
+# Add this folder to the python sys path; GDB Python-interpreter will now find modules in this path
+import sys
+from os import path
+self_dir = path.dirname(path.realpath(__file__))
+sys.path.append(self_dir)
+
 import gdb
 import gdb_lookup
-gdb_lookup.register_printers(gdb.current_objfile())
+
+# current_objfile can be none; even with `gdb foo-app`; sourcing this file after gdb init now works
+try:
+    gdb_lookup.register_printers(gdb.current_objfile())
+except Exception:
+    gdb_lookup.register_printers(gdb.selected_inferior().progspace)