Always fetch Ada "main" name from the executable

The gdb.ada/file-then-restart.exp test was failing with gnat-llvm.  I
tracked this down to the "main" name not being stored in a readonly
section, meaning that the code in ada_main_name using trust_readonly
did not work.

However, it seems to me that gdb should always prefer the data from
the executable in this particular case.  So, rather than relying on
trust_readonly, this patch changes gdb to do this directly.

Approved-By: Pedro Alves <pedro@palves.net>