[llvm-exegesis] Fix builds due to relanding #76368

Relanding this patch broke some builds (including Windows) due to
certain functions not being guarded by appropriate preprocessor
directives, particularly the loadImmediateSegmentRegister function not
having most of its functionality only enabled on Linux. The previous
relanding addressed issues with headers not being available on
non-x86_64 linux, but neglected to fix issues with the header not being
included, but the function still trying to use it on certain platforms,
such as x86-64 windows.