[lld][RISCV][test] Expand testing in riscv-attributes.s

This patch is related to the discussion in
<https://discourse.llvm.org/t/rfc-resolving-issues-related-to-extension-versioning-in-risc-v/68472/1>.
It slightly expands the coverage of behaviour for unrecognized
extensions (there are slightly different forms of error for zfoo vs e.g.
'y'), and adds coverage for an extension with an
unrecognized/unsupported version number.

Use "unrecognized" terminology because the expectation is the error
checking will be reduced in a follow-up patch posted for review.