Trouble linking Apple Frameworks about buck2

[zjturner]

So I figured out that I was using the frameworks argument wrong, I need to specify "$SDKROOT/System/Frameworks/AVFoundation.framework" and then it works. But I think that's just "happens" to fix the problem because it's a builtin location that the prelude has firsthand knowledge of. the problem above would still arise for other frameworks that aren't in the -isysroot location. So my primary question is solved, but I'd like to leave this open to possibly get some context about the design of the rule hierarchy, and whether we can support providing an AppleToolchainInfo directly from the _cxx_toolchain. I made this work locally FWIW but will hold off with the PR until I have some clarity on the team's desired direction.