MIPS:remove redundant #ifdef ARCH_MIPS64

there exist redundant #ifdef ARCH_MIPS64 in asm.h, remove it

libcpu/mips/common/asm.h

@@ -261,11 +261,7 @@ symbol		=	value
 #define LONG_SRAV	srav
 
 #define LONG		.word
-#ifdef ARCH_MIPS64
-#define LONGSIZE	8
-#else
 #define LONGSIZE	4
-#endif
 #define LONGMASK	3
 #define LONGLOG		2