symfile, vdso: remove target sections

Target sections added by the add-symbol-file-from-memory command are not
removed when the process exits.  In fact, they are not removed, at all.

This causes GDB to crash in gdb.base/break-interp.exp.

Change the owner of those target sections to the object file generated in
symbol_file_add_from_memory and generalize the free_objfile observer in
symfile.c to remove target sections of any freed object file.