fuzz: Enable stack sentinel for fuzz builds

Enable Zephyr stack sentinel checks for the native libFuzzer build.
This complements the heap hardening option by catching Zephyr thread
stack overflows closer to the corrupting input.

CONFIG_STACK_SENTINEL: stores a magic value at the lowest addresses of
each thread stack and checks it on context switch, interrupt return,
k_yield(), and thread exit. When the sentinel is corrupted the system
traps immediately, giving the fuzzer a clear crash signal instead of
allowing silent corruption that manifests later in an unrelated path.
This is particularly useful in UBSan-only fuzz runs where ASan stack
redzones are not available.

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>

Author: tmleman

app/boards/native_sim_libfuzzer.conf

@@ -6,6 +6,7 @@ CONFIG_EXCEPTION_DEBUG=y
 CONFIG_ARCH_POSIX_TRAP_ON_FATAL=y
 CONFIG_SYS_HEAP_BIG_ONLY=y
 CONFIG_SYS_HEAP_HARDENING_EXTREME=y
+CONFIG_STACK_SENTINEL=y
 CONFIG_ZEPHYR_NATIVE_DRIVERS=y
 CONFIG_ARCH_POSIX_LIBFUZZER=y
 CONFIG_ZEPHYR_POSIX_FUZZ_TICKS=100