rustc_target: Add convenience functions for adding linker arguments

They ensure that lld and non-lld linker flavors get the same set of arguments

compiler/rustc_target/src/spec/powerpc_unknown_linux_musl.rs

@@ -3,7 +3,7 @@ use crate::spec::{LinkerFlavor, Target, TargetOptions};
 
 pub fn target() -> Target {
     let mut base = super::linux_musl_base::opts();
-    base.pre_link_args.entry(LinkerFlavor::Gcc).or_default().push("-m32".into());
+    base.add_pre_link_args(LinkerFlavor::Gcc, &["-m32"]);
     base.max_atomic_width = Some(32);
 
     Target {