Constrain the macho-stabs test added in f72d001e to run on systems...

Constrain the macho-stabs test added in f72d001e to run on systems configured with an x86 backend.

Summary: This fixes a failure on the Builder clang-cmake-armv7-quick bot.

Reviewers: lhames, jhenderson

Reviewed By: lhames

Subscribers: kristof.beyls, rupprecht, seiya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71792