Use run-time page size where it matters

In AArch64, pages may be 4K or 64K depending on kernel configuration.
The GNU C Library documentation suggests [1], "the correct interface
to query about the page size is sysconf". Introduce one new
architecture-specific function-like macro, page_size(), that on x86
and AArch32 remains a constant so as to minimally affect performance,
but on AArch64 is sysconf(_SC_PAGESIZE) for correctness.

1. https://www.gnu.org/software/libc/manual/html_node/Query-Memory-Parameters.html



To minimize churn, the PAGE_SIZE macro is left as a build-time
estimation of what the run-time page size might be.

This fixes the following errors for CRIU on AArch64 kernels with
CONFIG_ARM64_64K_PAGES=y, allowing dump of
`setsid sleep < /dev/null &> /dev/null` to succeed.

Error (kerndat.c:48): Can't stat self map_files: No such file or directory

Error (util.c:668): Can't read pme for pid 90: No such file or directory

Error (parasite-syscall.c:1135): Can't open 89/map_files/0x3ffb7da0000-0x3ffb7dac000 on procfs: No such file or directory

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>