[llvm] [SPARC][IAS] Add `illtrap` alias for `unimp` (PR #105928)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

It looks like GNU as doesn't allow it when targeting V8:
```
test.s: Assembler messages:
test.s:2: Error: Architecture mismatch on "illtrap ".
test.s:2: (Requires v9|v9a|v9b|v9c|v9d|v9e|v9v|v9m|m8; requested architecture is v8.)
```


https://github.com/llvm/llvm-project/pull/105928