Allow SysV-i386 ABI on everything other than Apple targets

Summary:
This matches other SysV ABIs that are different on Apple and non-Apple targets,
like `ABISysV_arm.cpp` for instance.

Reviewers: clayborg, emaste

Subscribers: aemerson, kristof.beyls, lldb-commits

Differential Revision: https://reviews.llvm.org/D39335

llvm-svn: 316673