Rollup merge of #48664 - Keruspe:codegen, r=alexcrichton

make codegen-backends directory name configurable

This allows to parallel-install several versions of rust system-wide
Fixes #48263

src/bootstrap/builder.rs

@@ -464,7 +464,7 @@ impl<'a> Builder<'a> {
 
     pub fn sysroot_codegen_backends(&self, compiler: Compiler) -> PathBuf {
         self.sysroot_libdir(compiler, compiler.host)
-            .with_file_name("codegen-backends")
+            .with_file_name(self.build.config.rust_codegen_backends_dir.clone())
     }
 
     /// Returns the compiler's libdir where it stores the dynamic libraries that