Add target_description_changed_p and target_get_tdep_info methods
    
    target_description_changed_p () is added as a new gdbarch function.
    Given a list of register values received from the inferior, it will check if
    the current target descriptor is no longer valid for the inferior. This is
    required because on SVE the register sizes can change whilst the inferior is
    running.
    
    target_get_tdep_info () is added as a new gdbarch function.
    Given a list of registers, it will return a tdep info which then can be used
    when creating/finding a valid target descriptor for that inferior.
    
    Include stubbed aarch64 versions.
    
    2018-09-13  Alan Hayward  <alan.hayward@arm.com>
    
    gdb/
    	* aarch64-tdep.c (aarch64_target_description_changed_p): New function.
    	(aarch64_target_get_tdep_info): New function.
    	(aarch64_gdbarch_init): Add in the new functions
    	* arch-utils.c (default_target_description_changed_p): New function.
    	(default_target_get_tdep_info): New function.
    	* arch-utils.h: (default_target_description_changed_p): New
    	declaration.
    	(default_target_get_tdep_info): New declaration.
    	* gdbarch.c: Regenerate.
    	* gdbarch.h: Regenerate.
    	* gdbarch.sh: (target_description_changed_p): New function.
    	(target_get_tdep_info): New function.