Improve RISC-V LD error message

Improve RISC-V LD error message

I recently ran into this error message and found it's not helpful: it
just tells me some temporary file can't be linked.  This slightly
improved one at least tells me it's because of an elf32/elf64 conflict.

	* elfnn-riscv.c (_bfd_riscv_elf_merge_private_bfd_data): Improve
	error message when linking elf32 and elf64.