[bigint] Add USE macro to silence "unused var" warnings

Bug: v8:11515
Change-Id: Idf92af0fe30efe5569e8aaaef1bcc364f1db8f59
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3000744
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75517}

src/bigint/bigint-internal.h

@@ -80,6 +80,8 @@ class ProcessorImpl : public Processor {
 #define DCHECK(cond) (void(0))
 #endif
 
+#define USE(var) ((void)var)
+
 // RAII memory for a Digits array.
 class Storage {
  public:

src/bigint/mul-karatsuba.cc

@@ -187,6 +187,7 @@ void ProcessorImpl::KaratsubaMain(RWDigits Z, Digits X, Digits Y,
   }
   // The intermediate result may have been bigger, but the final result fits.
   DCHECK(overflow == 0);  // NOLINT(readability/check)
+  USE(overflow);
 }
 
 }  // namespace bigint