Don't emit vendor attribute section if there is no attribute to emit.

	2019-01-16  Kito Cheng  <kito@andestech.com>
	bfd/
	* elf-attrs.c (vendor_obj_attr_size): Return 0 if size is 0 even
	for OBJ_ATTR_PROC.
	gas/
	* testsuite/gas/riscv/attribute-empty.d: New.