Switch RISC-V to large code model on LLVM 20

Given llvm/llvm-project#70308 is in LLVM 20, I guess we can switch to it barring any bugs.

Author: Zentrik

src/jitlayers.cpp

@@ -1328,9 +1328,11 @@ namespace {
         if (TheTriple.isAArch64())
             codemodel = CodeModel::Small;
         else if (TheTriple.isRISCV()) {
-            // RISC-V will support large code model in LLVM 21
-            // https://github.com/llvm/llvm-project/pull/70308
+#if JL_LLVM_VERSION >= 200000
+            codemodel = CodeModel::Large;
+#else
             codemodel = CodeModel::Medium;
+#endif
         }
         // Generate simpler code for JIT
         Reloc::Model relocmodel = Reloc::Static;