[libc] Fix temporary types for fputil::split template. (#163813)

Fix #163711.

Author: lntue

libc/src/__support/FPUtil/double_double.h

@@ -77,9 +77,9 @@ LIBC_INLINE constexpr NumberPair<T> split(T a) {
   NumberPair<T> r{0.0, 0.0};
   // CN = 2^N.
   constexpr T CN = static_cast<T>(1 << N);
-  constexpr T C = CN + 1.0;
-  double t1 = C * a;
-  double t2 = a - t1;
+  constexpr T C = CN + T(1);
+  T t1 = C * a;
+  T t2 = a - t1;
   r.hi = t1 + t2;
   r.lo = a - r.hi;
   return r;