LoongArch: Change SHMLBA from SZ_64K to PAGE_SIZE

LoongArch has hardware page coloring for L1 Cache, so we don't have
cache aliases. But SFB (Store Fill Buffer) still has aliases. So we
define SHMLBA to SZ_64K previously. But there are losts of applications
use PAGE_SIZE rather than SHMLBA to mmap() file pages and shared pages.
Of course we can fix them one by one, but not easy.

On the other hand, we can simply disable SFB for 4KB page size to fix
cache alias (there will be performance decrease, but acceptable), and
in future we will fix SFB in hardware. So we can safely define SHMLBA to
PAGE_SIZE (use the generic shmparam.h) to make life easier.

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