[binutils-gdb] Improve RISC-V LD error message

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 96b0927de3ebdb302d8d571c43da3db5ec23847e ***

Author: Palmer Dabbelt <palmer@dabbelt.com>
Branch: master
Commit: 96b0927de3ebdb302d8d571c43da3db5ec23847e

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.