Fix gdb.ada/import.exp when using mold

Fix gdb.ada/import.exp when using mold

We found that the gdb.ada/import.exp test fails when 'mold' is used as
the linker.  This happens because mold decides to mark most of the
symbols in the executable as being file-local.  I tend to think this
choice, while non-traditional, is probably fine.  So, this patch fixes
the problem by changing the relevant Ada code to look for file-local
symbols as well.

Furthermore, there are two overloads of lookup_minimal_symbol_linkage
that both have a final 'bool' parameter -- but with radically
different meanings.  This patch somewhat clears up this confusion as
well.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31378