Handle PRFM in AArch64 process record

Handle PRFM in AArch64 process record

This patch fixes the bug of handling PRFM instruction.  PRFM is documented
in a table with other load and store instructions, but it doesn't do any
load or store.  This patch also adds a unit test to PRFM instruction.

gdb:

2017-03-23  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_process_record_test): Declare.
	(_initialize_aarch64_tdep): Register it.
	(aarch64_record_load_store): Handle PRFM instruction.
	(aarch64_process_record_test): New function.