libstdc++: Make std::random_device throw more std::system_error [PR105081]

In r14-289-gf9412cedd6c0e7 I made the std::random_device constructor
throw std::system_error for unrecognized tokens. But it still throws
std::runtime_error for a token such as "rdseed" that is recognized but
not supported at runtime by the CPU the program is running on.

With this change we throw std::system_error for those cases too. This
fixes the following failures on Intel CPUs withour rdseed support:

FAIL: 26_numerics/random/random_device/94087.cc execution test
FAIL: 26_numerics/random/random_device/cons/token.cc execution test
FAIL: 26_numerics/random/random_device/entropy.cc execution test

libstdc++-v3/ChangeLog:

	PR libstdc++/105081
	* src/c++11/random.cc (random_device::_M_init): Throw
	std::system_error when the requested device is a valid token but
	not available at runtime.

libstdc++-v3/src/c++11/random.cc

@@ -373,6 +373,15 @@ namespace std _GLIBCXX_VISIBILITY(default)
 				      "(const std::string&):"
 				      " unsupported token"));
 
+#if defined ENOSYS
+    const int unsupported = ENOSYS;
+#elif defined ENOTSUP
+    const int unsupported = ENOTSUP;
+#else
+    const int unsupported = 0;
+#endif
+    int err = 0;
+
 #ifdef _GLIBCXX_USE_CRT_RAND_S
     if (which & rand_s)
     {
@@ -407,6 +416,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	      return;
 	    }
 	}
+      err = unsupported;
     }
 #endif // USE_RDSEED
 
@@ -427,6 +437,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	      return;
 	    }
 	}
+      err = unsupported;
     }
 #endif // USE_RDRAND
 
@@ -438,6 +449,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	    _M_func = &__ppc_darn;
 	    return;
 	  }
+	err = unsupported;
       }
 #endif // USE_DARN
 
@@ -458,6 +470,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	    _M_func = &__libc_getentropy;
 	    return;
 	  }
+	err = unsupported;
       }
 #endif // _GLIBCXX_HAVE_GETENTROPY
 
@@ -477,6 +490,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
       if (_M_file)
 	return;
 #endif // USE_POSIX_FILE_IO
+      err = errno;
     }
 #endif // _GLIBCXX_USE_DEV_RANDOM
 
@@ -493,9 +507,12 @@ namespace std _GLIBCXX_VISIBILITY(default)
     }
 #endif
 
-    std::__throw_runtime_error(
+    auto msg = __N("random_device::random_device(const std::string&):"
-	__N("random_device::random_device(const std::string&):"
+		   " device not available");
-	    " device not available"));
+    if (err)
+      std::__throw_syserr(err, msg);
+    else
+      std::__throw_runtime_error(msg);
 #endif // USE_MT19937
   }