Require cross compiler toolchain when cross compiling

[Gold856]

Description

Because native-utils marks toolchains as optional by default, it's possible to request a cross compilation and then... just have it not cross compile. This marks Linux cross compilers as non-optional if they are requested.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why, including events that led to this PR
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with all settings going back to the previous seasons's last release (seasons end after champs ends)
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added
• If this PR adds a dependency, the license has been checked for compatibility and steps taken to follow it

[Gold856]

Preferring to resolve via wpilibsuite/native-utils#21. PR will be updated when that is fixed.