firmware: fix builds with gcc stack smashing protection enabled

On Ubuntu 20.04 when builiding dpkg packages, even when cross-compiling
firmware, gcc stack smashing protection is enabled.  Let's provide what
is minimally required in order to sucessfully complete builds on such
platforms.

Change-Id: Ic2f68f16b0730e7b5db17c30effc29a2909d1997
Closes: OS#4687

firmware/libcommon/source/stack_check.c

@@ -0,0 +1,14 @@
+#include <stdint.h>
+#include <osmocom/core/panic.h>
+
+/* This is what's minimally required to fix builds on Ubuntu 20.04,
+ * where stack smashing protection is enabled by default when using dpkg
+ * - even when cross-compiling: https://osmocom.org/issues/4687
+ */
+
+uintptr_t __stack_chk_guard = 0xdeadbeef;
+
+void __stack_chk_fail(void)
+{
+	osmo_panic("Stack smashing detected!\r\n");
+}