[gn build] Add host_cpu=arm64 & current_os=linux => aarch64-unknown-linux-gnu

I've been using this triple in development for a while without issues,
it's passing check-llvm and check-clang.

(The above is the commit message, but the build is currently broken since
D114639, I intend to submit this once it's passing again and it's accepted in
review)

Differential Revision: https://reviews.llvm.org/D119331