fix build failure for sh4a due to missing colon in asm statement

Due to a missing ":" in an asm() statement, the "memory" clobber is
considered by gcc as an input operand and not a clobber, which causes a
build failure.

arch/sh/atomic_arch.h

@@ -21,7 +21,7 @@ static inline int a_sc(volatile int *p, int v)
 #define a_barrier a_barrier
 static inline void a_barrier()
 {
-	__asm__ __volatile__ ("synco" : : "memory");
+	__asm__ __volatile__ ("synco" ::: "memory");
 }
 
 #define a_pre_llsc a_barrier