LibC: Expose AK::timing_safe_compare via `timingsafe_memcmp(..)`

Ports / other userland often needs such an implementation to function.
Lets expose `AK::timing_safe_compare` under the same name used used in
OpenBSD / FreeBSD / Solaris and other projects.

Userland/Libraries/LibC/string.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021-2022, Brian Gianforcaro <bgianf@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -121,6 +122,11 @@ int memcmp(const void* v1, const void* v2, size_t n)
     return 0;
 }
 
+int timingsafe_memcmp(const void* b1, const void* b2, size_t len)
+{
+    return AK::timing_safe_compare(b1, b2, len) ? 1 : 0;
+}
+
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/memcpy.html
 void* memcpy(void* dest_ptr, const void* src_ptr, size_t n)
 {

Userland/Libraries/LibC/string.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021-2022, Brian Gianforcaro <bgianf@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -23,6 +24,7 @@ int strcmp(const char*, const char*);
 int strncmp(const char*, const char*, size_t);
 
 int memcmp(const void*, const void*, size_t);
+int timingsafe_memcmp(const void*, const void*, size_t);
 void* memcpy(void*, const void*, size_t);
 void* memmove(void*, const void*, size_t);
 void* memchr(const void*, int c, size_t);