[libc] Fix BigInt's operator %= (llvm#98484)

This patch fixes cases where we try to do var %= 1. Previously this operator was calling .div directly since it would perform the inplace division and return the remainder, however, as an early exit condition a division by one returns zero as the remainder. The remainder being returned by div was not being assigned to var.

Author: mikhailramalho

libc/src/__support/big_int.h

@@ -732,7 +732,8 @@ struct BigInt {
   }
 
   LIBC_INLINE constexpr BigInt operator%=(const BigInt &other) {
-    return *this->div(other);
+    *this = *this % other;
+    return *this;
   }
 
   LIBC_INLINE constexpr BigInt &operator*=(const BigInt &other) {