This hierarchy supports Linux systems using the new
asm-generic/unistd.h, which removes many familiar old syscalls.  For
example, to implement open(), newer Linux architectures require glibc
to invoke the __NR_openat syscall with AT_FDCWD.  This hierarchy
provides all those implementations.

It also provides support for 32-bit platforms using the 64-bit kernel
syscall APIs, as the 32-bit ones are no longer provided.  Note that
newer ILP32 environments (x32 or AArch64:ILP32, for example) are
converting to use more 64-bit types in kernel syscalls, so that aspect
of this support is in more flux as of this writing.