[rootless] Do not use libroot when targeting the simulator

[leptos-null]

What does this implement/fix? Explain your changes.

libroot is not compiled for the simulator.
To match this availability, use the fallback rootless macros when compiling for the simulator.

Checklist

• New code follows the code rules
• I've added modulemaps for any new libraries (e.g. see libactivator/module.modulemap): it should be possible to @import MyLibrary; in ObjC, or import MyLibrary in Swift.
• My contribution is code written by myself from reverse-engineered headers, licensed into the Public Domain as per LICENSE.md; or, code written by myself / taken from an existing project, released under an OSI-approved license, and I've added relevant licensing credit to LICENSE.md

Does this close any currently open issues?

No

Any relevant logs, error output, etc?

Any other comments?

I have not completed testing for this, which is why it's in draft and the PR is not complete.

Where has this been tested?

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