RISC-V: Give error for RVE PLTs.

RISC-V: Give error for RVE PLTs.

	bfd/
	* elfnn-riscv.c (riscv_make_plt_header): New arg output_bfd.  Change
	return type to bfd_boolean.  If	EF_RISCV_RVE call _bfd_error_handler
	and return FALSE.  Return TRUE at end.
	(riscv_make_plt_entry): Likewise.
	(riscv_elf_finish_dynamic_symbol): Update call to riscv_make_plt_entry.
	(riscv_elf_finish_dynamic_sections): Update call to
	riscv_make_plt_header.