gdb/python: preserve identity for gdb.Symtab objects

This commit changes symtab_to_symtab_object() so that each it is called
with a particular struct symtab * it returns the very same gdb.Symtab
object.

This is done by searching per-objfile linked list of instances and - if
found - return it rather than creating new gdb.Symtab.

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