[X86] Add initialization of MXCSR in llvm-exegesis

Summary: This patch is used to initialize the new added register MXCSR.

Reviewers: craig.topper, RKSimon

Subscribers: tschuett, courbet, llvm-commits, LiuChen3

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70874