[meta] Morph a few pub into pub(crate), and remove dead code;

cranelift/codegen/meta/src/isa/arm64/mod.rs

@@ -45,7 +45,7 @@ fn define_registers() -> IsaRegs {
     regs.build()
 }
 
-pub fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
+pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
     let settings = define_settings(&shared_defs.settings);
     let regs = define_registers();