[cmake] Only run llvm-codesign if targetting apple on an apple host

Summary:
Other platforms don't have the capability to perform llvm_codesign
step. If LLVM_CODESIGNING_IDENTITY is set then this chunk of code would
attempt to codesign if the target was Apple. But when cross compiling
to Darwin from Linux, for example, this step would fail. So test if the
host is Apple as well.

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64942

llvm-svn: 366498