[llvm-exegesis] Fix ifdefs in X86/Target.cpp

As discussed in https://reviews.llvm.org/D96498, the correct macro
to check together with _MSC_VER is _M_X64. Testing for _MSC_VER with
__x86_64__ made tests fail on Windows in https://reviews.llvm.org/D146301.

Additionally, this commit replaces a few calls to llvm_unreachable() with
report_fatal_error() because it is actually reached even on Linux/i686,
causing obscure segfaults when expanded to __builtin_unreachable().

~~

Huawei RRI, OS Lab

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D147926