cranelift: Merge abi and abi_impl modules (#4805)

cranelift/codegen/src/isa/s390x/abi.rs

@@ -1,6 +1,6 @@
 //! Implementation of a standard S390x ABI.
 //!
-//! This machine uses the "vanilla" ABI implementation from abi_impl.rs,
+//! This machine uses the "vanilla" ABI implementation from abi.rs,
 //! however a few details are different from the description there:
 //!
 //! - On s390x, the caller must provide a "register save area" of 160
@@ -575,7 +575,7 @@ impl ABIMachineSpec for S390xMachineDeps {
     }
 
     // Returns stack bytes used as well as instructions. Does not adjust
-    // nominal SP offset; abi_impl generic code will do that.
+    // nominal SP offset; abi generic code will do that.
     fn gen_clobber_save(
         _call_conv: isa::CallConv,
         _setup_frame: bool,