[llvm-exegesis] Ensure that ExecutableFunction are aligned.

Summary: Experiments show that this is the alignment we get (for ELF+Linux), but let's ensure that we have it.

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68703

llvm-svn: 374170