RISC-V: Allow pcrel_lo addends, error on addend overflow.

	bfd/
	* elfnn-riscv.c (riscv_resolve_pcrel_lo_relocs): Add check for reloc
	overflow with addend.  Use reloc_dangerous instead of reloc_overflow.
	Add strings for the two errors handled here.
	(riscv_elf_relocate_section) In case R_RISCV_PCREL_LO12_I, rewrite
	comment.  Only give error with addend when used with section symbol.
	In case bfd_reloc_dangerous, update error string.

	ld/
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Run pcrel-lo-addend-2.
	* testsuite/ld-riscv/elf/ld-riscv-elf/pcrel-lo-addend-2.d: New.
	* testsuite/ld-riscv/elf/ld-riscv-elf/pcrel-lo-addend-2.s: New.
	* testsuite/ld-riscv/elf/ld-riscv-elf/pcrel-lo-addend.d: Update name
	and error string.