Userland: static vs non-static constexpr variables

Problem:
- `static` variables consume memory and sometimes are less
  optimizable.
- `static const` variables can be `constexpr`, usually.
- `static` function-local variables require an initialization check
  every time the function is run.

Solution:
- If a global `static` variable is only used in a single function then
  move it into the function and make it non-`static` and `constexpr`.
- Make all global `static` variables `constexpr` instead of `const`.
- Change function-local `static const[expr]` variables to be just
  `constexpr`.

Userland/Libraries/LibC/time.cpp

@@ -70,10 +70,9 @@ char* ctime_r(const time_t* t, char* buf)
     return asctime_r(localtime_r(t, &tm_buf), buf);
 }
 
-static const int __seconds_per_day = 60 * 60 * 24;
-
 static void time_to_tm(struct tm* tm, time_t t)
 {
+    constexpr int __seconds_per_day = 60 * 60 * 24;
     int year = 1970;
     for (; t >= days_in_year(year) * __seconds_per_day; ++year)
         t -= days_in_year(year) * __seconds_per_day;