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/x86_64_pc_solaris.rs

@@ -2,7 +2,7 @@ use crate::spec::{LinkerFlavor, SanitizerSet, StackProbeType, Target};
 
 pub fn target() -> Target {
     let mut base = super::solaris_base::opts();
-    base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-m64".into()]);
+    base.add_pre_link_args(LinkerFlavor::Gcc, &["-m64"]);
     base.cpu = "x86-64".into();
     base.vendor = "pc".into();
     base.max_atomic_width = Some(64);