Reland 293e8fa13d3f05e993771577a4c022deee5cbf6e

Reland 293e8fa13d3f05e993771577a4c022deee5cbf6e
    [llvm-exegesis] Disable the LBR check on AMD

    https://bugs.llvm.org/show_bug.cgi?id=48918

    The bug reported a hang (or very very slow runtime) on a Zen2. Unfortunately, we don't have the hardware right now to debug it and I was not able to reproduce the bug on a HSW.
    Theory we've got is that the lbr-checking code could be confused on AMD.

    Differential Revision: https://reviews.llvm.org/D97504

New change:
 - Surround usages of x86 helper in llvm-exegesis/X86/Target.cpp with ifdef
 - Fix bug which caused the caller of getVendorSignature to not have a copy of EAX that it expected.