ARC: MWDT: libc: add _exit support

Default weak _exit implementation from ARC MWDT libs
calls _exit_halt from startup libs. As we are going to
get rid of startup libs usage let's implement _exit
stub.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>

lib/libc/arcmwdt/libc-hooks.c

@@ -76,3 +76,9 @@ int *___errno(void)
 {
 	return z_errno();
 }
+
+__weak void _exit(int status)
+{
+	while (1) {
+	}
+}