linux_syscall_support.h: add aarch64 support
Aarch64 support for linux_syscall_support.h. Since Aarch64 is a brand
new architecture, none of the legacy system calls are neccesarily
available. Thus some changes were neccesary affect other architectures
1) use getdents64 where available and else getdents (for ppc64)
2) other legacy system calls, pipe, waitpid and open replaced by pipe2,
wait4 and openat where available.
3) use fstatat if stat is not available.
The aarch64 system call interface follows the Aarch64 calling convention
(regs x0-x5 for arguments and x8 system call number - return in x0).
Clone implementation is adapted from glibc.
v2: step back in getdents removal due to ppc64