Default to the medium code model in x86 linux (JuliaLang#53391)

This shouldn't have any cost on smaller images because the only thing
that gets put into ldata is the system image data, which is only
reference via `dlsym`. This allows for images larger than 2gb (tested by
putting a 2gb array in the base image)

I did not test how this might be handled in other platforms (Windows
doesn't support it).

src/aotcompile.cpp

@@ -4,6 +4,7 @@
 #include "platform.h"
 
 // target support
+#include "llvm/Support/CodeGen.h"
 #include <llvm/ADT/Triple.h>
 #include <llvm/ADT/Statistic.h>
 #include <llvm/Analysis/TargetLibraryInfo.h>
@@ -1607,10 +1608,11 @@ void jl_dump_native_impl(void *native_code,
     if (TheTriple.isOSLinux() || TheTriple.isOSFreeBSD()) {
         RelocModel = Reloc::PIC_;
     }
+
     CodeModel::Model CMModel = CodeModel::Small;
-    if (TheTriple.isPPC()) {
-        // On PPC the small model is limited to 16bit offsets
-        CMModel = CodeModel::Medium;
+    if (TheTriple.isPPC() || (TheTriple.isX86() && TheTriple.isArch64Bit() && TheTriple.isOSLinux())) {
+        // On PPC the small model is limited to 16bit offsets. For very large images the small code model
+        CMModel = CodeModel::Medium; //  isn't good enough on x86 so use Medium, it has no cost because only the image goes in .ldata
     }
     std::unique_ptr<TargetMachine> SourceTM(
         jl_ExecutionEngine->getTarget().createTargetMachine(
@@ -1653,6 +1655,12 @@ void jl_dump_native_impl(void *native_code,
                                      GlobalVariable::ExternalLinkage,
                                      data, "jl_system_image_data");
         sysdata->setAlignment(Align(64));
+#if JL_LLVM_VERSION >= 180000
+        sysdata->setCodeModel(CodeModel::Large);
+#else
+        if (TheTriple.isX86() && TheTriple.isArch64Bit() && TheTriple.isOSLinux())
+            sysdata->setSection(".ldata");
+#endif
         addComdat(sysdata, TheTriple);
         Constant *len = ConstantInt::get(sysimgM.getDataLayout().getIntPtrType(Context), z->size);
         addComdat(new GlobalVariable(sysimgM, len->getType(), true,