arch POSIX: provide a bottom which uses natsim's thread emul

This allows building with embedded libCs in the Zephyr side,
as the POSIX arch bottom is not anymore built in Zephyr context.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

arch/posix/core/CMakeLists.txt

@@ -6,13 +6,21 @@ zephyr_library_sources(
 	cpuhalt.c
 	fatal.c
 	irq.c
-	nsi_compat/nsi_compat.c
-	nsi_compat/nce.c
-	posix_core.c
 	swap.c
 	thread.c
 	)
 
-zephyr_include_directories(
-	nsi_compat/
-)
+if(CONFIG_NATIVE_APPLICATION)
+	zephyr_include_directories(
+		nsi_compat/
+	)
+	zephyr_library_sources(
+		posix_core.c
+		nsi_compat/nsi_compat.c
+		nsi_compat/nce.c
+	)
+else()
+	zephyr_library_sources(
+		posix_core_nsi.c
+	)
+endif()

arch/posix/core/posix_core_nsi.c

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * Interfacing between the POSIX arch and the Native Simulator (nsi) CPU thread emulator
+ *
+ * This posix architecture "bottom" will be used when building with the native simulator.
+ */
+
+#include "nct_if.h"
+
+static void *te_state;
+
+/*
+ * Initialize the posix architecture
+ */
+void posix_arch_init(void)
+{
+	extern void posix_arch_thread_entry(void *pa_thread_status);
+	te_state = nct_init(posix_arch_thread_entry);
+}
+
+/*
+ * Clear the state of the POSIX architecture
+ * free whatever memory it may have allocated, etc.
+ */
+void posix_arch_clean_up(void)
+{
+	nct_clean_up(te_state);
+}
+
+void posix_swap(int next_allowed_thread_nbr, int this_th_nbr)
+{
+	(void) this_th_nbr;
+	nct_swap_threads(te_state, next_allowed_thread_nbr);
+}
+
+void posix_main_thread_start(int next_allowed_thread_nbr)
+{
+	nct_first_thread_start(te_state, next_allowed_thread_nbr);
+}
+
+int posix_new_thread(void *payload)
+{
+	return nct_new_thread(te_state, payload);
+}
+
+void posix_abort_thread(int thread_idx)
+{
+	nct_abort_thread(te_state, thread_idx);
+}
+
+int posix_arch_get_unique_thread_id(int thread_idx)
+{
+	return nct_get_unique_thread_id(te_state, thread_idx);
+}