RISC-V: Give error for ignored pcrel_lo addend.

RISC-V: Give error for ignored pcrel_lo addend.

	bfd/
	* elfnn-riscv.c (riscv_elf_relocate_section): Use bfd_reloc_dangerous
	when pcrel_lo reloc has an addend.  Use reloc_dangerous callback for
	bfd_reloc_dangerous.  Use einfo instead of warning callback for errors.
	Add %X%P to error messages.

	ld/
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Run pcrel-lo-addend test.
	* testsuite/ld-riscv-elf/pcrel-lo-addend.d: New.
	* testsuite/ld-riscv-elf/pcrel-lo-addend.s: New.