On ppc64/aarch64 Linux can be set to use Large pages, so the PAGE_SIZE
isn't build-time constant anymore. Define it through _SC_PAGESIZE.

There are different sizes for a page on ppc64:
: #if defined(CONFIG_PPC_256K_PAGES)
: #define PAGE_SHIFT              18
: #elif defined(CONFIG_PPC_64K_PAGES)
: #define PAGE_SHIFT              16
: #elif defined(CONFIG_PPC_16K_PAGES)
: #define PAGE_SHIFT              14
: #else
: #define PAGE_SHIFT              12
: #endif

And on aarch64 there are default sizes and possibly someone can set his
own PAGE_SHIFT:
: config ARM64_PAGE_SHIFT
:         int
:         default 16 if ARM64_64K_PAGES
:         default 14 if ARM64_16K_PAGES
:         default 12

On the downside - each time we need PAGE_SIZE, we're doing libc
function call on aarch64/ppc64.

Fixes: #415
Tested-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>