Make ecall inline asm as clobbering memory, which acts as a compiler barrier.

sdk/include/stdlib.h

diff --git a/sdk/include/stdlib.h b/sdk/include/stdlib.h
index 6955d23..adc2143 100644
--- a/sdk/include/stdlib.h
+++ b/sdk/include/stdlib.h
@@ -329,6 +329,6 @@
 
 static inline void yield(void)
 {
-	__asm volatile("ecall");
+	__asm volatile("ecall" ::: "memory");
 }
 __END_DECLS