win: fix uv_available_parallelism on win32 (libuv#4525)

Fixes commit 58dfb6c from a few days ago. DWORD_PTR is 32 bits on
x86 Windows. Use the right bit count when checking the population count.

Interestingly enough, it manifested itself as double counting online
processors, presumably because the compiler emits a ROR instead of SHR.

Fixes: libuv#4524

Author: bnoordhuis

src/win/util.c

@@ -516,7 +516,7 @@ unsigned int uv_available_parallelism(void) {
    */
   count = 0;
   if (GetProcessAffinityMask(GetCurrentProcess(), &procmask, &sysmask))
-    for (i = 0; i < 64; i++)  /* a.k.a. count = popcount(procmask); */
+    for (i = 0; i < 8 * sizeof(procmask); i++)
       count += 1 & (procmask >> i);
 
   if (count > 0)