kernel: fatal: Fix NO_OPTIMIZATIONS build

When logging is on and optimization and multithreading is off then
build fails to link because unoptimized compiler/linker seems to not
look beyond the function and it fails trying to link k_thread_name_get.
Reworking the code to make it known to the compiler without optimization
that k_thread_name_get is not needed and not logging current thread
name in that case.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit 28b4bab)

kernel/fatal.c

@@ -110,8 +110,9 @@ void z_fatal_error(unsigned int reason, const struct arch_esf *esf)
 	}
 #endif /* CONFIG_ARCH_HAS_NESTED_EXCEPTION_DETECTION */
 
-	LOG_ERR("Current thread: %p (%s)", thread,
-		thread_name_get(thread));
+	if (IS_ENABLED(CONFIG_MULTITHREADING)) {
+		LOG_ERR("Current thread: %p (%s)", thread, thread_name_get(thread));
+	}
 
 	coredump(reason, esf, thread);