Define RB_GC_GUARD in a way it cannot be an assignment LHS

* Because the form `RB_GC_GUARD(a) = b` is not supported on TruffleRuby,
  as we need the value to be set before RB_GC_GUARD() is called.

Author: eregon

lib/cext/ABI_version.txt

@@ -1 +1 @@
-2
+3

lib/cext/include/ruby/internal/memory.h

@@ -158,12 +158,10 @@ typedef uint128_t DSIZE_T;
  * @post   `v` is still alive.
  */
 #ifdef TRUFFLERUBY
-#define RB_GC_GUARD(v) \
-    (*__extension__ ({ \
-       polyglot_invoke(RUBY_CEXT, "rb_tr_gc_guard", v); \
-       volatile VALUE *rb_gc_guarded_ptr = &v; \
-       rb_gc_guarded_ptr; \
-    }))
+RBIMPL_SYMBOL_EXPORT_BEGIN()
+VALUE rb_tr_gc_guard(VALUE value);
+RBIMPL_SYMBOL_EXPORT_END()
+#define RB_GC_GUARD(v) rb_tr_gc_guard(v)
 #else
 #ifdef __GNUC__
 #define RB_GC_GUARD(v) \

src/main/c/cext/gc.c

@@ -12,6 +12,11 @@
 
 // GC, rb_gc_*
 
+VALUE rb_tr_gc_guard(VALUE value) {
+  polyglot_invoke(RUBY_CEXT, "rb_tr_gc_guard", value);
+  return value;
+}
+
 void rb_global_variable(VALUE *address) {
   rb_gc_register_address(address);
 }