Add instruction_reader to arm process record

Add instruction_reader to arm process record

This patch adds an abstract class abstract_memory_reader a
and pass it to the code reading instructions in arm process record,
rather than using target_read_memory to read from real target.  This
paves the way for adding more unit tests to arm process record.

gdb:

2017-03-16  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (abstract_memory_reader): New class.
	(instruction_reader): New class.
	(extract_arm_insn): Add argument 'reader'.  Callers updated.
	(decode_insn): Likewise.