gdbarch: add addressable_memory_unit_size method

gdbarch: add addressable_memory_unit_size method

Add a new gdbarch method to get the length of an addressable memory unit
for a given architecture. The default implementation returns 1.

gdb/ChangeLog:

	* arch-utils.h (default_addressable_memory_unit_size): New.
	* arch-utils.c (default_addressable_memory_unit_size): New.
	* gdbarch.sh (addressable_memory_unit_size): New.
	* gdbarch.h: Re-generate.
	* gdbarch.c: Re-generate.