[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.

llvm/tools/llvm-exegesis/lib/X86/Target.cpp

@@ -929,6 +929,9 @@ void generateSyscall(long SyscallNumber, std::vector<MCInst> &GeneratedCode) {
 constexpr std::array<unsigned, 6> SyscallArgumentRegisters{
     X86::RDI, X86::RSI, X86::RDX, X86::R10, X86::R8, X86::R9};
 
+// The functions below for saving and restoring system call registers are only
+// used when llvm-exegesis is built on Linux.
+#ifdef __linux__
 static void saveSyscallRegisters(std::vector<MCInst> &GeneratedCode,
                                  unsigned ArgumentCount) {
   assert(ArgumentCount <= 6 &&
@@ -956,10 +959,11 @@ static void restoreSyscallRegisters(std::vector<MCInst> &GeneratedCode,
   generateRegisterStackPop(X86::R11, GeneratedCode);
   generateRegisterStackPop(X86::RCX, GeneratedCode);
 }
+#endif // __linux__
 
 static std::vector<MCInst> loadImmediateSegmentRegister(unsigned Reg,
                                                         const APInt &Value) {
-#ifdef __x86_64__
+#if defined(__x86_64__) and defined(__linux__)
   assert(Value.getBitWidth() <= 64 && "Value must fit in the register.");
   std::vector<MCInst> loadSegmentRegisterCode;
   // Preserve the syscall registers here as we don't
@@ -986,7 +990,7 @@ static std::vector<MCInst> loadImmediateSegmentRegister(unsigned Reg,
 #else
   llvm_unreachable("Loading immediate segment registers is only supported with "
                    "x86-64 llvm-exegesis");
-#endif
+#endif // defined(__x86_64__) and defined(__linux__)
 }
 
 std::vector<MCInst> ExegesisX86Target::setRegTo(const MCSubtargetInfo &STI,