[kernel][ktl] Update for upstream libc++ <cwchar> changes

This fixes the kernel's stubbed-out <cwchar> header to match
the new internal API expected by recent upstream libc++ changes.

Fixed: 117187
Change-Id: I132e8baa91d5b60a9a9d308b809c98c5b48c0788
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/774684
Reviewed-by: Haowei Wu <haowei@google.com>
Fuchsia-Auto-Submit: Roland McGrath <mcgrathr@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>

Author: frobtech

zircon/kernel/lib/ktl/include/cwchar

@@ -20,6 +20,23 @@ using ::wmemcpy;
 using ::wmemmove;
 using ::wmemset;
 
+// These are called by libc++ header code that should never be instantiated.
+
+constexpr size_t __constexpr_wcslen(const wchar_t* str) {
+  __builtin_abort();
+  return 0;
+}
+
+constexpr int __constexpr_wmemcmp(const wchar_t* lhs, const wchar_t* rhs, size_t count) {
+  __builtin_abort();
+  return -1;
+}
+
+constexpr const wchar_t* __constexpr_wmemchr(const wchar_t* str, wchar_t wc, size_t count) {
+  __builtin_abort();
+  return nullptr;
+}
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // ZIRCON_KERNEL_LIB_KTL_INCLUDE_CWCHAR