Auto merge of #51590 - bjorn3:codegen_llvm_extract, r=alexcrichton

Mostly fix metadata_only backend and extract some code out of rustc_codegen_llvm

Removes dependency on the `ar` crate and removes the `llvm.enabled` config option in favour of setting `rust.codegen-backends` to `[]`.

src/test/run-make-fulldeps/hotplug_codegen_backend/Makefile

@@ -4,6 +4,6 @@ all:
 	/bin/echo || exit 0 # This test requires /bin/echo to exist
 	$(RUSTC) the_backend.rs --crate-name the_backend --crate-type dylib \
 		-o $(TMPDIR)/the_backend.dylib
-	$(RUSTC) some_crate.rs --crate-name some_crate --crate-type bin -o $(TMPDIR)/some_crate \
+	$(RUSTC) some_crate.rs --crate-name some_crate --crate-type lib -o $(TMPDIR)/some_crate \
 		-Z codegen-backend=$(TMPDIR)/the_backend.dylib -Z unstable-options
 	grep -x "This has been \"compiled\" successfully." $(TMPDIR)/some_crate

src/test/run-make-fulldeps/hotplug_codegen_backend/some_crate.rs

@@ -8,6 +8,5 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() {
-    ::std::process::exit(1);
-}
+#![feature(no_core)]
+#![no_core]

src/test/run-make-fulldeps/hotplug_codegen_backend/the_backend.rs

@@ -63,7 +63,7 @@ impl CodegenBackend for TheBackend {
         let crate_name = ongoing_codegen.downcast::<Symbol>()
             .expect("in join_codegen_and_link: ongoing_codegen is not a Symbol");
         for &crate_type in sess.opts.crate_types.iter() {
-            if crate_type != CrateType::CrateTypeExecutable {
+            if crate_type != CrateType::CrateTypeRlib {
                 sess.fatal(&format!("Crate type is {:?}", crate_type));
             }
             let output_name =