bc: declare interntional unsafe fixnum arithmetic

Eliminate overflow errors from fixnum test when racket bc compiled with
--enable-ubsan (issue #2314)

.github/workflows/ci-ubsan.yml

@@ -84,7 +84,9 @@ jobs:
       continue-on-error: true
       run: raco test -l tests/db/all-tests | tee logs/db-all-tests.log
     - name: Gather runtime errors
-      run: grep 'runtime error' logs/*.log > runtime-errors_git${{ github.sha }}.log
+      run: |
+        grep 'runtime error' logs/*.log > runtime-errors_git${{ github.sha }}.log || true
+        test ! -s runtime-errors_git${{ github.sha }}.log
     - uses: actions/upload-artifact@v2
       with:
         name: runtime-errors_git${{ github.sha }}

racket/src/bc/src/numarith.c

@@ -1228,7 +1228,14 @@ static Scheme_Object *fx_abs(int argc, Scheme_Object *argv[])
   return o;
 }
 
-#define UNSAFE_FX(name, op, fold, zero_args, PRE_CHECK)                \
+#if __GNUC__ >= 8 || __clang_major__ >= 4
+# define NO_SANITIZE_SIGNED_INTEGER_OVERFLOW \
+  __attribute__((no_sanitize("signed-integer-overflow")))
+#else
+# define NO_SANITIZE_SIGNED_INTEGER_OVERFLOW
+#endif
+#define UNSAFE_FX(name, op, fold, zero_args, PRE_CHECK)                 \
+ NO_SANITIZE_SIGNED_INTEGER_OVERFLOW                                    \
  static Scheme_Object *name(int argc, Scheme_Object *argv[]) \
  {                                                           \
    intptr_t v;                                                          \