LibJS+LibJIT: Let users of JIT::Assembler handle caller-saved registers

Instead of JIT::Assembler making the decision for everyone and forcing
out every caller-saved register in the ABI onto the stack, we now leave
that decision to users of JIT::Assembler.

Userland/Libraries/LibJS/JIT/Compiler.h

@@ -129,6 +129,8 @@ private:
 
     void jump_to_exit();
 
+    void native_call(void* function_address, Vector<Assembler::Operand> const& stack_arguments = {});
+
     template<typename Codegen>
     void branch_if_int32(Assembler::Reg, Codegen);