[llvm-objcopy] If input=output, preserve umask bits, otherwise drop S_ISUID/S_ISGID bits

This makes the behavior similar to cp

```
chmod u+s,g+s,o+x a
sudo llvm-strip a -o b
// With this patch, b drops set-user-ID and set-group-ID bits.
// sudo cp a b => b does not have set-user-ID or set-group-ID bits.
```

This also changes the behavior for the following case:

```
chmod u+s,g+s,o+x a
llvm-strip a
// a preserves set-user-ID and set-group-ID bits.
// This matches binutils<2.36 and probably >=2.37.  2.36 and 2.36.1 have some compatibility issues.
```

Differential Revision: https://reviews.llvm.org/D97253