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 librari

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

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