libc: minimal: declare sig_atomic_t and signal()

We don't actually implement signal() in Zephyr, but some users
require the declaration.

Additionally, typedef sig_atomic_t since it is part of ISO.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

lib/libc/minimal/CMakeLists.txt

@@ -10,6 +10,7 @@ set(STRERROR_TABLE_H ${GEN_DIR}/libc/minimal/strerror_table.h)
 zephyr_library_cc_option(-fno-builtin)
 
 zephyr_library_sources(
+  source/signal/signal.c
   source/stdlib/atoi.c
   source/stdlib/strtol.c
   source/stdlib/strtoul.c

lib/libc/minimal/include/signal.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_LIB_LIBC_MINIMAL_INCLUDE_SIGNAL_H_
+#define ZEPHYR_LIB_LIBC_MINIMAL_INCLUDE_SIGNAL_H_
+
+#define SIG_DFL ((sighandler_t)0)
+#define SIG_ERR ((sighandler_t)1)
+#define SIG_IGN ((sighandler_t)-1)
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef int	sig_atomic_t;		/* Atomic entity type (ANSI) */
+
+/* Note: sighandler_t is a gnu-ism, but it simplifies the declaration below */
+typedef void (*sighandler_t)(int signo);
+
+sighandler_t signal(int signum, sighandler_t handler);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  /* ZEPHYR_LIB_LIBC_MINIMAL_INCLUDE_SIGNAL_H_ */

lib/libc/minimal/source/signal/signal.c

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2023 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <signal.h>
+
+sighandler_t signal(int signum, sighandler_t handler)
+{
+	return SIG_DFL;
+}