[llvm-exegesis] Set up AsmTargetStreamer in readSnippets

Summary: This is a follow up on D71137 properly setting up the AsmTargetStreamer prior to AsmParser::Run call.

Reviewers: courbet, mstojanovic

Subscribers: tschuett, mikhail.ramalho, llvm-commits, petarj, atanasyan

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71468