Avoid adding fastcall attribute on linux aarch64 (#1282)

fastcall should only be added for 32 bit x86, as explained in the
comment. On some linux aarch64 compilers, none of the existing defines
are set, and __aarch64__ is set instead. Adding this avoids warnings
like:

```
In file included from /home/runner/haxelib/hxcpp/git/include/hxcpp.h:336:
/home/runner/haxelib/hxcpp/git/include/hx/Object.h:60:65: warning: ‘fastcall’ attribute directive ignored [-Wattributes]
   60 | typedef void (CPPIA_CALL *StackExecute)(struct StackContext *ctx);
      |                                                                 ^
In file included from /home/runner/haxelib/hxcpp/git/include/hxcpp.h:336:
/home/runner/haxelib/hxcpp/git/include/hx/Object.h:60:65: warning: ‘fastcall’ attribute directive ignored [-Wattributes]
   60 | typedef void (CPPIA_CALL *StackExecute)(struct StackContext *ctx);
      |                                                                 ^
```

Author: tobil4sk

include/hx/Object.h

@@ -42,7 +42,7 @@ HXCPP_EXTERN_CLASS_ATTRIBUTES null BadCast();
 // CPPIA_CALL = fastcall on x86(32), nothing otherwise
 #if (defined(_WIN32) && !defined(_M_X64) && !defined(__x86_64__) && !defined(_ARM_) ) || \
    defined(HXCPP_X86) || defined(__i386__) || defined(__i386) || \
-   (!defined(_WIN32) && !defined(_ARM_) && !defined(__arm__) && !defined(__x86_64__) )
+   (!defined(_WIN32) && !defined(_ARM_) && !defined(__arm__) && !defined(__aarch64__) && !defined(__x86_64__) )
 
    #if defined(__GNUC__) && !defined(__APPLE__) && !defined(EMSCRIPTEN)
       #define CPPIA_CALL __attribute__ ((fastcall))