[llvm] [gn build] Allow specifying macOS deployment target (PR #107492)

[Nico Weber via llvm-commits]

nico wrote:

I think it's probably fine. Not a ton of people use the gn build, even fewer on macOS. I'd try upping the default to 12, and if it happens to break someone, they can shout and we can add an arg then.. I'm guessing everyone using this is on 12 by now though.

https://github.com/llvm/llvm-project/pull/107492