Move bignum code path testing out of the library

Without this, it's not at all obvious that turning on MBEDTLS_TEST_HOOKS
doesn't change the functional behavior of the code.

Signed-off-by: Janos Follath <janos.follath@arm.com>

library/bignum_core_invasive.h

@@ -0,0 +1,23 @@
+/**
+ * \file bignum_core_invasive.h
+ *
+ * \brief Function declarations for invasive functions of bignum core.
+ */
+/**
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#ifndef MBEDTLS_BIGNUM_CORE_INVASIVE_H
+#define MBEDTLS_BIGNUM_CORE_INVASIVE_H
+
+#include "bignum_core.h"
+
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
+
+extern void (*mbedtls_safe_codepath_hook)(void);
+extern void (*mbedtls_unsafe_codepath_hook)(void);
+
+#endif /* MBEDTLS_TEST_HOOKS && !MBEDTLS_THREADING_C */
+
+#endif /* MBEDTLS_BIGNUM_CORE_INVASIVE_H */