[compiler-rt] [Compiler-rt] Add AArch64 routines for __arm_agnostic("sme_za_state") (PR #120059)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Is that expected? Are we holding something wrong? Or is that too little information to say something useful?

I don't think it's expected (looks like the chrome build is clang trunk).

It looks like the `+sme2` detection code is not working:
```c++
// Fails to build (+sme2 not set)
void foo(void)  __arm_streaming_compatible {
  asm(".arch armv9-a+sme2");
  asm("smstart");
  asm("ldr zt0, [sp]");
}
```
See: https://godbolt.org/z/osdfGd6dn

---

It looks like this would work:
```c++
__attribute__((target("arch=armv9-a+sme2")))
void foo(void)  __arm_streaming_compatible {
  asm("smstart");
  asm("ldr zt0, [sp]");
}
```
See: https://godbolt.org/z/Y4h86ovK9

https://github.com/llvm/llvm-project/pull/120059