[mach-o] Fix stub generation to work for dylibs and bundles

Split up the CRuntimeFile into one part for output types that need an entry
point and another part for output types that use stubs.

Add file 'test/mach-o/Inputs/libSystem.yaml' for use by test cases that
use -dylib and therefore may now need the helper symbol in libSystem.dylib.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@215602 91177308-0d34-0410-b5e6-96231b3b80d8