gdb: change find_pcs_for_symtab_line() to return entries instead of PCs

This commit changes find_pcs_for_symtab_line() to return complete
linetable entries instead of just PCs.  This is a preparation for adding
more attributes to gdb.LinetableEntry objects.

I also renamed the function to find_linetable_entries_for_symtab_line()
to better reflect what it does.

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