gdb: introduce expanded_symbols_functions

This commit adds new "quick" symbol functions, expanded_symbols_functions,
that work purely on examining objfile's compunits. This is useful for
example for JIT reader API where symbols are created by user-provided
code.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33554
Approved-By: Tom Tromey <tom@tromey.com>