[aarch64] Handle unknown instruction encoding

[aarch64] Handle unknown instruction encoding

We see the following build error:

 gdb/aarch64-tdep.c: In function 'aarch64_process_record':
 gdb/aarch64-tdep.c:2823:17: error: 'record_buf[0]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
                 memcpy(&REGS[0], &RECORD_BUF[0], sizeof(uint32_t)*LENGTH); \
                 ^
 gdb/aarch64-tdep.c:3529:12: note: 'record_buf[0]' was declared here
   uint32_t record_buf[2];

since record_buf[0] isn't initialized in some paths in which
instruction encodings doesn't exist in current ARMv8 reference manual.
This patch is to let the function return AARCH64_RECORD_UNKNOWN for
these paths.

gdb:

2015-05-27  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_record_data_proc_simd_fp): Return
	AARCH64_RECORD_UNKNOWN for unknown instruction encoding.