Also fix sub_with_borrow.

Author: gchatelet

libc/src/__support/math_extras.h

@@ -100,7 +100,7 @@ add_with_carry(T a, T b, T carry_in, T &carry_out) {
 template <typename T>
 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
 sub_with_borrow(T a, T b, T carry_in, T &carry_out) {
-  if constexpr (!cpp::is_constant_evaluated()) {
+  if (!cpp::is_constant_evaluated()) {
 #if __has_builtin(__builtin_subcb)
     RETURN_IF(unsigned char, __builtin_subcb)
 #elif __has_builtin(__builtin_subcs)