Add new gdbarch method, read_core_file_mappings

Add new gdbarch method, read_core_file_mappings

The new gdbarch method, read_core_file_mappings, will be used for
reading file-backed mappings from a core file.  It'll be used
for two purposes: 1) to construct a table of file-backed mappings
in corelow.c, and 2) for display of core file mappings.

For Linux, I tried a different approach in which knowledge of the note
format was placed directly in corelow.c.  This seemed okay at first;
it was only one note format and the note format was fairly simple.
After looking at FreeBSD's note/mapping reading code, I concluded
that it's best to leave architecture specific details for decoding
the note in (architecture specific) tdep files.

With regard to display of core file mappings, I experimented with
placing the mappings display code in corelow.c.  It has access to the
file-backed mappings which were read in when the core file was loaded.
And, better, still common code could be used for all architectures.
But, again, the FreeBSD mapping code convinced me that this was not
the best approach since it has even more mapping info than Linux.
Display code which would work well for Linux will leave out mappings
as well as protection info for mappings.

So, for these reasons, I'm introducing a new gdbarch method for
reading core file mappings.

gdb/ChangeLog:

	* arch-utils.c (default_read_core_file_mappings): New function.
	* arch-utils.c (default_read_core_file_mappings): Declare.
	* gdbarch.sh (read_core_file_mappings): New gdbarch method.
	* gdbarch.h, gdbarch.c: Regenerate.