[RISC-V] Avoid using an allocated extension name in invalid-attributes.s
The Y extension has been allocated and will no longer trigger an error
once https://github.com/llvm/llvm-project/pull/176870 lands. Use 't' to
test this case instead which is still marked as reserved and as far as I
know is not currently reserved for any future extensions.
Pull Request: https://github.com/llvm/llvm-project/pull/177094