Have S3 distribution follow Linux architecture naming conventions

Related to #30, it's unnecessarily difficult to build things like Docker containers which install the session plugin because the URLs have arbitrary naming conventions for the architecture:

https://s3.amazonaws.com/session-manager-downloads/plugin/latest/linux_arm64/session-manager-plugin.rpm
https://s3.amazonaws.com/session-manager-downloads/plugin/latest/linux_64bit/session-manager-plugin.rpm

Ideally we'd have YUM and APT repositories as requested in #30 but for this particular need it would be nice if the architecture names matched Linux's architecture names so this could be as simple as:

```docker
RUN yum install -y https://s3.amazonaws.com/session-manager-downloads/plugin/latest/linux_$(uname -m)/session-manager-plugin.rpm
```

where they'd be published as aarch64 or x86_64.

Currently, I have this less than ideal comparison:

```docker
RUN yum install -y https://s3.amazonaws.com/session-manager-downloads/plugin/latest/linux_$(if [ $(uname -m) == "aarch64" ]; then echo "arm64"; else echo "64bit"; fi)/session-manager-plugin.rpm
```