[VCRUNTIME] Implement purecall handler support

Implements _get_purecall_handler, _set_purecall_handler and _purecall

sdk/lib/vcruntime/CMakeLists.txt

···
     ${vcrt_runtime_asm}
     __std_terminate.c
     __vcrt_init.c
+    purecall.cpp
 )
 
 # Runtime library (linked into ucrtbase)

sdk/lib/vcruntime/purecall.cpp

···
+/*
+ * PROJECT:     ReactOS vcruntime library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Implementation of _get_purecall_handler, _set_purecall_handler and _purecall function
+ * COPYRIGHT:   Copyright 2025 Timo Kreuzer <[e-mail]>
+ */
+
+#include <stdlib.h>
+#include <intrin.h>
+
+static volatile _purecall_handler purecall_handler;
+
+extern "C"
+_purecall_handler _cdecl _get_purecall_handler(void)
+{
+    return purecall_handler;
+}
+
+extern "C"
+_purecall_handler _cdecl _set_purecall_handler(_In_opt_ _purecall_handler _Handler)
+{
+    return reinterpret_cast<_purecall_handler>(
+        _InterlockedExchangePointer(reinterpret_cast<void* volatile*>(&purecall_handler),
+                                    reinterpret_cast<void*>(_Handler)));
+}
+
+extern "C"
+int __cdecl _purecall(void)
+{
+    _purecall_handler handler = purecall_handler;
+    if (handler)
+        handler();
+    abort();
+}