fix memory corruption in old GCC

This typo introduced memory corruption when __builtin_add_overflow
is not available but uint128_t is.  GCC before 5 are one of such
situatins.

See also https://rubyci.org/logs/rubyci.s3.amazonaws.com/opensuseleap/ruby-master/log/20191009T120004Z.log.html.gz

bootstraptest/test_insns.rb

@@ -427,7 +427,6 @@ tests.compact.each do |(insn, expr, *a)|
   end
 end
 
-
 # with trace
 tests.compact.each {|(insn, expr, *a)|
   progn = "set_trace_func(proc{})\n" + expr

gc.c

@@ -135,7 +135,7 @@ size_add_overflow(size_t x, size_t y)
 
 #elif defined(DSIZE_T)
     RB_GNUC_EXTENSION DSIZE_T dx = x;
-    RB_GNUC_EXTENSION DSIZE_T dy = x;
+    RB_GNUC_EXTENSION DSIZE_T dy = y;
     RB_GNUC_EXTENSION DSIZE_T dz = dx + dy;
     p = dz > SIZE_MAX;
     z = (size_t)dz;