Switch to using orc::ObjectLinkingLayer and a smaller code model on…

… Darwin

This change is inspired by this comment [0], switching our linking layer
to the newer one as recommended by [1].  With these changes, I can pass
the `Distributions` test suite on aarch64 darwin, and so it appears it
fixes at least one of the segfault issues noted on apple silicon.

[0] #41440 (comment)
[1] https://llvm.org/docs/JITLink.html#jitlink-and-objectlinkinglayer

src/codegen.cpp

@@ -8092,11 +8092,14 @@ extern "C" void jl_init_llvm(void)
     }
     // Allocate a target...
     Optional<CodeModel::Model> codemodel =
-#ifdef _P64
+#if defined(_OS_DARWIN_) && defined(_CPU_AARCH64_)
+        // Let LLVM pick its default code model on aarch64 darwin
+        None;
+#elif defined(_P64)
         // Make sure we are using the large code model on 64bit
-        // Let LLVM pick a default suitable for jitting on 32bit
         CodeModel::Large;
 #else
+        // Let LLVM pick a default suitable for jitting on 32bit
         None;
 #endif
     auto optlevel = CodeGenOptLevelFor(jl_options.opt_level);

src/jitlayers.h

@@ -169,7 +169,11 @@ class JuliaOJIT {
 #endif
 
 public:
+#if defined(_OS_DARWIN_)
     typedef orc::RTDyldObjectLinkingLayer ObjLayerT;
+#else
+    typedef orc::ObjectLinkingLayer ObjLayerT;
+#endif
     typedef orc::IRCompileLayer CompileLayerT;
 #if JL_LLVM_VERSION < 120000
     typedef RTDyldObjHandleT ModuleHandleT;