tools/loongarch: Use __SIZEOF_LONG__ to define __BITS_PER_LONG

Although __SIZEOF_POINTER__ is equal to _SIZEOF_LONG__ on LoongArch,
it is better to use __SIZEOF_LONG__ to define __BITS_PER_LONG to keep
consistent between arch/loongarch/include/uapi/asm/bitsperlong.h and
tools/arch/loongarch/include/uapi/asm/bitsperlong.h.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>