[lld-macho] Downgrade missing fat arch to warning

This mirrors ld64's behavior. In many cases this likely still leads to a
link failure but if you didn't actually use anything from the library
it can be ignored. If you care about these invalid cases -fatal_warnings
still upgrades it back to an error.

Example: https://github.com/keith/ld64.lld/issues/3

Differential Revision: https://reviews.llvm.org/D141638