EC2: Add new "small" AMIs

These are the same as the standard "base" images except:

• They don't have kernel or world debug symbols,
• They don't have FreeBSD tests,
• They don't have 32-bit libraries,
• They don't have LLDB,
• They don't have the Amazon SSM Agent pre-installed,
• They don't default to installing the awscli at first boot.

This reduces the amount of disk space in use when the EC2 instance
finishes booting from ~5 GB to ~1 GB.

Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46509