[llvm-exegesis] InMemoryAssembler: handle return-less targets (e.g. arm).

Summary: Arm does not have a ret code per se.

Reviewers: gchatelet

Subscribers: mgorny, javed.absar, kristof.beyls, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D45672

llvm-svn: 332331