[clang][driver] set macOS as the target OS for -arch arm64 when clang is running on an Apple Silicon mac
This change allows users to use -arch arm64 to build for mac when
running it on Apple Silicon mac without explicit -target option.
Differential Revision: https://reviews.llvm.org/D82428
(cherry picked from commit 565603cc94d79a8d0de6df840fd53714899fb890)