LibC: Implement strerror_r()

This implements the XSI-compliant version of strerror_r() - as opposed
to the GNU-specific variant.

The function explicitly saves errno so as to not accidentally change it
with one of the calls to other functions.

Tests/LibC/TestLibCString.cpp

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <errno.h>
+#include <string.h>
+
+TEST_CASE(strerror_r_basic)
+{
+    EXPECT_EQ(strerror_r(1000, nullptr, 0), EINVAL);
+    EXPECT_EQ(strerror_r(EFAULT, nullptr, 0), ERANGE);
+    char buf[64];
+    EXPECT_EQ(strerror_r(EFAULT, buf, sizeof(buf)), 0);
+    EXPECT_EQ(strcmp(buf, "Bad address"), 0);
+}