gdb/python: allow instantiation of gdb.Symbol from Python

gdb/python: allow instantiation of gdb.Symbol from Python

This commit adds code to allow user extension to instantiate
gdb.Symbol.

As of now only "function" symbols can be created (that is: symbols
of FUNCTION_DOMAIN and with address class LOC_BLOCK). This is enough
to be able to implement "JIT reader" equivalent in Python. Future
commits may extend this API to allow creation of other kinds of symbols
(static variables, arguments, locals and so on).

Like previous similar commits, this is a step towards a Python support
for dynamically generated code (JIT) in GDB.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>