libstdc++: Simplify test by not using std::log2

This test uses std::log2 without including <cmath>, but it doesn't need
to use it at all. Just get the number of digits from numeric_limits
instead.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/random/random_device/entropy.cc: Use
	numeric_limits<unsigned>::digits.

libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc

@@ -1,6 +1,7 @@
 // { dg-do run { target c++11 } }
 
 #include <random>
+#include <limits>
 #include <testsuite_hooks.h>
 #include <testsuite_random.h>
 
@@ -12,7 +13,7 @@ test01()
       VERIFY( std::random_device(token).entropy() == 0.0 );
 
   using result_type = std::random_device::result_type;
-  const double max = std::log2(std::numeric_limits<result_type>::max() + 1.0);
+  const double max = std::numeric_limits<result_type>::digits;
 
   for (auto token : { "/dev/random", "/dev/urandom" })
     if (__gnu_test::random_device_available(token))