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

[via llvm-commits]

koachan 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.)
> ```

Oh, interesting. Seems like using `unimp` in V9 is fine but using `illtrap` in V8 is not.

On the other hand Solaris does allow both mnemonics even when explicitly targeting V8:
```
$ cat unimp.s
u:
 unimp
 unimp 0
 illtrap
 illtrap 0
 retl
  nop
$ as -m32 -xarch=v8 unimp.s && dis unimp.o
disassembly for unimp.o


section .text
    0x0:                    00 00 00 00  unimp     0x0
    0x4:                    00 00 00 00  unimp     0x0
    0x8:                    00 00 00 00  unimp     0x0
    0xc:                    00 00 00 00  unimp     0x0
    0x10:                   81 c3 e0 08  retl
    0x14:                   01 00 00 00  nop
```
I'm slightly leaning towards using Solaris behavior in this particular case but in any case either choice is okay by me.

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