[compiler-rt] [compiler-rt][ubsan][nfc-ish] Fix a type conversion bug (PR #100665)

[Anton Korobeynikov via llvm-commits]

asl wrote:

@alanzhao1 Actually, the spec for `ptrauth_sign` is:
```
   The result will have the same type as the original value.
```

So, I'd think we also fix the inline asm version while here. How about the following:

```
      __typeof(__value) ret;                \
```
?

https://github.com/llvm/llvm-project/pull/100665