aarch64: split emit tests into its own file;

This is done to satisfy a check done on the maximal file's size when
vendoring Rust source code into Mozilla central's repository.

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -26,6 +26,9 @@ pub use self::args::*;
 pub mod emit;
 pub use self::emit::*;
 
+#[cfg(test)]
+mod emit_tests;
+
 //=============================================================================
 // Instructions (top level): definition