feat: add freebsd support

[rickystewart]

Type of change

• New feature or functionality (change which adds functionality)

For changes visible to end-users

• Breaking change (this change will force users to change their own code or config)
• Relevant documentation has been updated
• Suggested release notes are provided below:

Add basic support for freebsd_amd64.

Test plan

• Manual testing; please provide instructions so we can reproduce:

Ran bazel build tools:{copy_to_directory,copy_directory}, confirmed that a FreeBSD executable is generated

[CLAassistant]

All committers have signed the CLA.

[alexeagle]

Thanks! I guess we'll have to verify these changes work properly when we cut the next release...