RISC-V: Stop reporting warnings for mismatched extension versions

The extension version checking logic is really just too complicated to
encode into the linker, trying to do so causes more harm than good.
This removes the checks and the associated tests, leaving the logic to
keep the largest version of each extension linked into the target.

bfd/

	* elfnn-riscv.c (riscv_version_mismatch): Rename to
	riscv_update_subset_version, and stop reporting warnings on
	version mismatches.
	(riscv_merge_std_ext): Adjust calls to riscv_version_mismatch.
	(riscv_merge_multi_letter_ext): Likewise.

ld/
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01.d: Remove
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01a.s: Likewise
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-01b.s: Likewise
	* testsuite/ld-riscv-elf/attr-merge-arch-failed-02.d: Likewise
	* 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/attr-merge-user-ext-01.d: New test.
	* testsuite/ld-riscv-elf/attr-merge-user-ext-rv32i21_m2p0.s:
	Likewise.
	* testsuite/ld-riscv-elf/attr-merge-user-ext-rv32i21_m2p1.s:
	Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Remove obselete
	attr-merge-arch-failed-{01,02}, replace with
	attr-merge-user-ext-01.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>