Update comment for struct type's length field, introduce type_length_units

Update comment for struct type's length field, introduce type_length_units

This patch tries to clean up a bit the blur around the length field in
struct type, regarding its use with architectures with non-8-bits
addressable memory.  It clarifies that the field is expressed in host
bytes, which is what is the closest to the current reality.

It also introduces a new function to get the length of the type in
target addressable memory units.

gdb/ChangeLog:

	* gdbtypes.c (type_length_units): New function.
	* gdbtypes.h (type_length_units): New declaration.
	(struct type) <length>: Update comment.