[llvm] [AArch64] Disable FP loads/stores when fp-armv8 not enabled (PR #77817)

[via llvm-commits]

ostannard wrote:

Since clang's `-mgeneral-regs-only` is already broken, and most FPU instructions are already rejected, I'd rather not revert this patch.

Would something like this work for you as a workaround? I've checked and this is accepted by both gcc and clang with `-mgeneral-regs-only`:

```
void foo() {
  __asm__(".arch_extension fp\n"
          "stp     q0, q1, [x8, #(0 * 32)]\n");
}
```

https://github.com/llvm/llvm-project/pull/77817