Check library name rather than member name when rereading symbols.

Check library name rather than member name when rereading symbols.

On Darwin, we have lots of complaints being emitted when restarting
a program:

    (gdb) start
    `a-except.o' has disappeared; keeping its symbols.
    `unwind-dw2.o' has disappeared; keeping its symbols.
    `s-except.o' has disappeared; keeping its symbols.
    `s-traceb.o' has disappeared; keeping its symbols.

These object files are part of the GNAT runtime, and were never available.
The warning comes from the fact that we're checking whether the .o files
in the GNAT shared runtime have changed whereas we should be checking
whether the GNAT shared library itself has changed.

This patch implements this.  Although it is really only useful on a platform
such as Darwin (debug info stored in .o files), we believe that this is
the right thing to do in general.  This change should be a noop for all
the other platforms in any case.

gdb/ChangeLog (from Tristan Gingold & Pedro Alves):

        * symfile.c (reread_symbols): Also search for file in libraries.
        Update comment.

Tested on x86_64-darwin and x86_64-linux.