Choose TARGET_OBJECT_STACK_MEMORY and TARGET_OBJECT_MEMORY in read_value_memory

Choose TARGET_OBJECT_STACK_MEMORY and TARGET_OBJECT_MEMORY in read_value_memory

Before this patch
https://sourceware.org/ml/gdb-patches/2014-02/msg00709.html
read_value_memory checks parameter 'stack', and call read_stack or
read_memory respectively.  However, 'stack' is not checked and
TARGET_OBJECT_MEMORY is always used in target_xfer_partial, which is
a mistake in the patch above.

This patch checks parameter 'stack', and choose TARGET_OBJECT_MEMORY
or TARGET_OBJECT_STACK_MEMORY accordingly.

gdb:

2016-04-22  Yao Qi  <yao.qi@linaro.org>

	* valops.c (read_value_memory): New local variable 'stack'.
	Set it to either TARGET_OBJECT_STACK_MEMORY or
	TARGET_OBJECT_MEMORY.