RISC-V: Improve the error message for the mis-matched ISA versions.

Consider the updated attr-merge-arch-failed-01.d testcase.  Extension
A's version are mis-matched between attr-merge-arch-failed-01a.s and
attr-merge-arch-failed-01b.s.  But the old binutils reports that the
mis-matched extension is M rather than A.  This commit is used to fix
the wrong mis-matched error message.

Besides, when parsing the arch string in the riscv_parse_subset, it
shouldn't be NULL or empty.  However, it might be empty when we failed
to merge the arch string in the riscv_merge_attributes.  Since we should
already issue the correct error message in another side, and the message
- ISA string must begin with rv32 or rv64 - is meaninglesss when the arch
string is empty, so do not issue it.

	bfd/
	* elfnn-riscv.c (riscv_merge_std_ext): Fix to report the correct
	error message when the versions of extension are mis-matched.
	* elfxx-riscv.c (riscv_parse_subset): Don't issue the error when
	the string is empty.

	ld/
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01.d: Updated.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01a.s: Likewise.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01b.s: Likewise.