Remove Linkage::Private

This is the same as Linkage::Internal except that it doesn't emit any
symbol. Some backends may not support it and it isn't all that useful
anyway.

compiler/rustc_codegen_gcc/src/base.rs

@@ -51,7 +51,6 @@ pub fn global_linkage_to_gcc(linkage: Linkage) -> GlobalKind {
         Linkage::WeakODR => unimplemented!(),
         Linkage::Appending => unimplemented!(),
         Linkage::Internal => GlobalKind::Internal,
-        Linkage::Private => GlobalKind::Internal,
         Linkage::ExternalWeak => GlobalKind::Imported, // TODO(antoyo): should be weak linkage.
         Linkage::Common => unimplemented!(),
     }
@@ -68,7 +67,6 @@ pub fn linkage_to_gcc(linkage: Linkage) -> FunctionType {
         Linkage::WeakODR => unimplemented!(),
         Linkage::Appending => unimplemented!(),
         Linkage::Internal => FunctionType::Internal,
-        Linkage::Private => FunctionType::Internal,
         Linkage::ExternalWeak => unimplemented!(),
         Linkage::Common => unimplemented!(),
     }