Replace two #ifs with #if/#else in new integer random algorithm (#25352)

Swift gets used on archs other than 32 and 64b Intel and ARM (e.g. IBM's systems); as written this would fail to compile there.

Author: stephentyrone

stdlib/public/core/Random.swift

@@ -104,8 +104,7 @@ extension RandomNumberGenerator {
     } while random < range
 
     return random % upperBound
-#endif
-#if arch(x86_64) || arch(arm64)
+#else
     var random: T = next()
     var m = random.multipliedFullWidth(by: upperBound)
     if m.low < upperBound {