Bug 1599465 - Part 6: Change test to check if uint128 types are available. r=jwalden

We never define `HAVE_INT128_SUPPORT`, so the uint128 code path is never
enabled. Change the test to use `__SIZEOF_INT128__` to enable it on
applicable platforms.

Differential Revision: https://phabricator.services.mozilla.com/D54762

--HG--
extra : moz-landing-system : lando

js/src/vm/BigIntType.cpp

@@ -251,7 +251,7 @@ BigInt* BigInt::neg(JSContext* cx, HandleBigInt x) {
 #if !defined(JS_64BIT)
 #  define HAVE_TWO_DIGIT 1
 using TwoDigit = uint64_t;
-#elif defined(HAVE_INT128_SUPPORT)
+#elif defined(__SIZEOF_INT128__)
 #  define HAVE_TWO_DIGIT 1
 using TwoDigit = __uint128_t;
 #endif