Have one version of libSystem for each arch. NFC.

An upcoming commit will change how we choose to reference a dylib.  Currently
dylibs are only given an LC_LOAD_DYLIB in the final image if an atom is used.
This is different from ld64 which adds the load command when the dylib is referenced
on the cmdline.

In order to change this behaviour, we need libSystem.yaml to actually contain a mach header
so that it is parsed as a dylib, instead of currently being parsed as a normalised file.

To get a mach header, we also require an arch, so now we have one libsystem per arch and
all the tests have been updated to choose the correct one.

llvm-svn: 278372