Change argument opcode type from enum aarch64_opcodes to uint32_t

Change argument opcode type from enum aarch64_opcodes to uint32_t

The patch fixes the following errors in C++ build,

gdb/gdbserver/linux-aarch64-low.c: In function 'int emit_data_processing(uint32_t*, aarch64_opcodes, aarch64_register, aarch64_register, aarch64_operand)':
gdb/gdbserver/linux-aarch64-low.c:1071:52: error: invalid conversion from 'unsigned int' to 'aarch64_opcodes' [-fpermissive]
       return emit_data_processing_reg (buf, opcode | operand_opcode, rd,
                                                    ^
gdb/gdbserver:

2015-11-19  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (emit_data_processing_reg): Change opcode
	type to uint32_t.