[llvm] [RISC-V] Avoid using an allocated extension name in invalid-attributes.s (PR #177094)

[Alexander Richardson via llvm-commits]

arichardson wrote:

Based on this comment: https://github.com/llvm/llvm-project/pull/176870#discussion_r2709461447

Looking at https://riscv.github.io/riscv-isa-manual/snapshot/privileged/#misa there are still a few marked as reserved. I picked T since I wasn't aware of any potential future uses but happy to use any other character.

https://github.com/llvm/llvm-project/pull/177094