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

[Nico Weber via llvm-commits]

nico wrote:

Fine with me. Which deployment target do you want? If it's, say, 11, I think we could instead just bump the default to that. 12 would be fine too I think. Up to you!

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