src/arch: use sysconf() for page size on aarch64/riscv64 libc builds

The /proc/self/auxv parsing silently falls back to 4096 if /proc isn't
mounted (chroots, minimal containers), which yields a wrong page size
on 16K/64K page kernels and hence broken ring size/offset calculations.
Only nolibc builds need the manual auxv parsing, use sysconf() when
building with a libc like the generic arch code does.

Signed-off-by: Jens Axboe <axboe@kernel.dk>