Remove Linkage::Appending

It can only be used for certain LLVM internal variables like
llvm.global_ctors which users are not allowed to define.

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -41,7 +41,6 @@ fn linkage_by_name(tcx: TyCtxt<'_>, def_id: LocalDefId, name: &str) -> Linkage {
     // ghost, dllimport, dllexport and linkonce_odr_autohide are not supported
     // and don't have to be, LLVM treats them as no-ops.
     match name {
-        "appending" => Appending,
         "available_externally" => AvailableExternally,
         "common" => Common,
         "extern_weak" => ExternalWeak,

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -190,7 +190,6 @@ fn prefix_and_suffix<'tcx>(
             Linkage::Internal => {
                 // write nothing
             }
-            Linkage::Appending => emit_fatal("Only global variables can have appending linkage!"),
             Linkage::Common => emit_fatal("Functions may not have common linkage"),
             Linkage::AvailableExternally => {
                 // this would make the function equal an extern definition