[gn build] Make an explicit `use_lld = true` on mac use lld.darwinnew

use_lld defaults to true on non-mac if clang_base_path is set (i.e.
the host compiler is a locally-built clang). On mac, the lld Mach-O
port used to be unusable, but ld64.lld.darwinnew is close to usable.
When explicitly setting `use_lld = true` in a GN build on a mac host,
check-lld passes, two check-clang tests fail, and a handful check-llvm
tests fail (the latter all due to -flat_namespace not yet being implemented).