Don't use .debug_names if .debug_aranges is missing

Don't use .debug_names if .debug_aranges is missing

gdb's .debug_names reader depends on .debug_aranges as well, because
if the ranges are missing, it won't be able to create a map from PC to
CU.

However, this requirement isn't enforced by gdb.  I no longer recall
know why, but I can see in commit b371f07c ("Rewrite .debug_names
reader") that I made this choice on purpose:

    In v1 of this patch, I made the new reader more strict about requiring
    .debug_aranges.  In v2, I've backed this out and kept the previous
    logic.  This solved a few test failures, though it's arguably not the
    right approach.

However, the current setup now seems clearly incorrect to me.  And, it
was causing problems for another patch I'm working on.

Fixing this required patching a few tests to add a .debug_aranges
section.  I've also updated the intro comment to
read_addrmap_from_aranges to reflect how it ought to be used.

Regression tested on x86-64 Fedora 41 with both the default and the
cc-with-debug-names target boards.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33847