RISC-V: Report warnings rather than errors for the mis-matched ISA versions.

Same as the privileged spec attributes check - different ISA versions
should be compatible, unless there are some known conflicts.  Therefore,
we should allow to link objects with different ISA versions, and update
the output ISA versions once the corresponding input ones are newer.
But it's better to also warn people that the conflicts may happen when
the ISA versions are mis-matched.

	bfd/
	* elfnn-riscv.c (riscv_version_mismatch): Change the return type
	from void to bfd_boolean.  Report warnings rather than errors
	when the ISA versions are mis-matched.  Afterwards, remember to
	update the output ISA versions to the newest ones.
	(riscv_merge_std_ext): Allow to link objects with different
	standard ISA versions.  Try to add output ISA versions to
	merged_subsets first.
	(riscv_merge_multi_letter_ext): Likewise.  But for standard additional
	ISA and non-standard ISA versions.

	ld/
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01.d: Update the
	message from error to warning.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-02.d: New testcases.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-02a.s: Likewise.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-02b.s: Likewise.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-02c.s: Likewise.
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-02d.s: Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.

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