[darwin][arm64] use the "cyclone" CPU for Darwin even when `-arch`

is not specified

The -target option allows the user to specify the build target using LLVM
triple. The triple includes the arch, and so the -arch option is redundant.
This should work just as well without the -arch. However, the driver has a bug
in which it doesn't target the "Cyclone" CPU for darwin if -target is used
without -arch. This commit fixes this issue.

rdar://46743182

Differential Revision: https://reviews.llvm.org/D55731

llvm-svn: 349382