gdb: Print compatible information within print_xml_feature
    
    The gdbsupport directory contains a helper class print_xml_feature
    that is shared between gdb and gdbserver.  This class is used for
    printing an XML representation of a target_desc object.
    
    Currently this class doesn't have the ability to print the
    <compatible> entities that can appear within a target description, I
    guess no targets have needed that functionality yet.
    
    The print_xml_feature classes API is based around operating on the
    target_desc class, however, the sharing between gdb and gdbserver is
    purely textural, we rely on their being a class called target_desc in
    both gdb and gdbserver, but there is no shared implementation.  We
    then have a set of functions declared that operate on an object of
    type target_desc, and again these functions have completely separate
    implementations.
    
    Currently then the gdb version of target_desc contains a vector of
    bfd_arch_info pointers which represents the compatible entries from a
    target description.  The gdbserver version of target_desc has no such
    information.  Further, the gdbserver code doesn't seem to include the
    bfd headers, and so doesn't know about the bfd types.
    
    I was reluctant to include the bfd headers into gdbserver just so I
    can reference the compatible information, which isn't (currently) even
    needed in gdbserver.
    
    So, the approach I take in this patch is to wrap the compatible
    information into a new helper class.  This class is declared in the
    gdbsupport library, but implemented separately in both gdb and
    gdbserver.
    
    In gdbserver the class is empty.  The compatible information within
    the gdbserver is an empty list, of empty classes.
    
    In gdb the class contains a pointer to the bfd_arch_info object.
    
    With this in place we can now add support to print_xml_feature for
    printing the compatible information if it is present.  In the
    gdbserver code this will never happen, as the gdbserver never has any
    compatible information.  But in gdb, this code will trigger when
    appropriate.
    
    gdb/ChangeLog:
    
    	* target-descriptions.c (class tdesc_compatible_info): New class.
    	(struct target_desc): Change type of compatible vector.
    	(tdesc_compatible_p): Update for change in type of
    	target_desc::compatible.
    	(tdesc_compatible_info_list): New function.
    	(tdesc_compatible_info_arch_name): New function.
    	(tdesc_add_compatible): Update for change in type of
    	target_desc::compatible.
    	(print_c_tdesc::visit_pre): Likewise.
    
    gdbserver/ChangeLog:
    
    	* tdesc.cc (struct tdesc_compatible_info): New struct.
    	(tdesc_compatible_info_list): New function.
    	(tdesc_compatible_info_arch_name): New function.
    
    gdbsupport/ChangeLog:
    
    	* tdesc.cc (print_xml_feature::visit_pre): Print compatible
    	information.
    	* tdesc.h (struct tdesc_compatible_info): Declare new struct.
    	(tdesc_compatible_info_up): New typedef.
    	(tdesc_compatible_info_list): Declare new function.
    	(tdesc_compatible_info_arch_name): Declare new function.