Build: Add LLVM toolchain support for an521 only.

Support for other platforms will follow.

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: Id036ecd78bf8e8909af30f78b2bfe5262b78c6b9
diff --git a/secure_fw/partitions/lib/runtime/crt_exit.c b/secure_fw/partitions/lib/runtime/crt_exit.c
new file mode 100644
index 0000000..1c80e66
--- /dev/null
+++ b/secure_fw/partitions/lib/runtime/crt_exit.c
@@ -0,0 +1,10 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ * SPDX-FileCopyrightText: Copyright The TrustedFirmware-M Contributors
+ */
+
+void exit(void)
+{
+    while (1)
+        ;
+}