target: consider addressable unit size when reading/writing memory

target: consider addressable unit size when reading/writing memory

If we are reading/writing from a memory object, the length represents
the number of "addresses" to read/write, so the addressable unit size
needs to be taken into account when allocating memory on gdb's side.

gdb/ChangeLog:

	* target.c (target_read): Consider addressable unit size when
	reading from a memory object.
	(read_memory_robust): Same.
	(read_whatever_is_readable): Same.
	(target_write_with_progress): Consider addressable unit size
	when writing to a memory object.
	* target.h (target_read): Update documentation.
	(target_write): Add documentation.