gdb/corelow.c: avoid repeated warnings in build_file_mappings

gdb/corelow.c: avoid repeated warnings in build_file_mappings

When GDB opens a coredump it tries to locate and then open all files
which were mapped in the process.

If a file is found but cannot be opened with BFD (bfd_open /
bfd_check_format fails), then a warning is printed to the user.  If the
same file was mapped multiple times in the process's address space, the
warning is printed once for each time the file was mapped.  I find this
un-necessarily noisy.

This patch makes it so the warning message is printed only once per
file.

There was a comment in the code assuming that if the file was found on
the system, opening it (bfd_open + bfd_check_format) should always
succeed.  A recent change in BFD (014a602b86f "Don't optimise bfd_seek
to same position") showed that this assumption is not valid.  For
example, it is possible to have a core dump of a process which had
mmaped an IO page from a DRI render node (/dev/dri/runderD$NUM).  In
such case the core dump does contain the information that portions of
this special file were mapped in the host process, but trying to seek to
position 0 will fail, making bfd_check_format fail.  This patch removes
this comment.

Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>