gdb/python: preserve identity for gdb.Symbol objects

gdb/python: preserve identity for gdb.Symbol objects

This commit changes symbol_to_symbol_object() so that each it is called
with a particular struct symbol * it returns the very same gdb.Symbol
object.

This is done in the same way as for gdb.Symtab objects in earlier commit
("gdb/python: preserve identity for gdb.Symtab objects") except that
symbols may be either objfile-owned or arch-owned.

Prior this commit, arch-owned objects we not put into any list (like
objfile-owned ones) so they could not be easily looked up. This commit
changes the code so arch-owned list are put into per-architecture list
which is then used (solely) for looking up arch-owned gdb.Symbol.

Approved-By: Tom Tromey <tom@tromey.com>