Update the aws-c-s3 submodule to the latest release

Update the `aws-c-s3` submodule to
[v0.8.5](https://github.com/awslabs/aws-c-s3/releases/tag/v0.8.5),
picking up in particular: [Move fulfilling pending future outside the lock and ignore already completed futures (#536)](awslabs/aws-c-s3#536).

Change details:
```
Submodule mountpoint-s3-crt-sys/crt/aws-c-s3 f8ae82e3..70aacd2d:
  > Move fulfilling pending future outside the lock and ignore already completed futures (#536)
```

Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>

Author: passaro