shared/base-filesystem: add define for arm64

shared/base-filesystem: add define for arm64

https://wiki.debian.org/ArchitectureSpecificsMemo:
> arm64 aarch64-linux-gnu 64 AARCH64 /lib/ld-linux-aarch64.so.1 aarch64 aarch64

Fedora:
$ ls -l /lib /lib64
lrwxrwxrwx. 1 root root 7 Jul 27  2020 /lib -> usr/lib
lrwxrwxrwx. 1 root root 9 Jul 27  2020 /lib64 -> usr/lib64
$ ldd /bin/sh|grep ld
/lib/ld-linux-aarch64.so.1 (0x0000ffff8c905000)
$ ls -l /lib/ld-linux-aarch64.so.1 /lib64/ld-2.32.so
lrwxrwxrwx. 1 root root     19 Jul 13 07:28 /lib/ld-linux-aarch64.so.1 -> ../lib64/ld-2.32.so
-rwxr-xr-x. 1 root root 961248 Jul 13 07:56 /lib64/ld-2.32.so

$ uname -r
5.14.16-101.fc33.aarch64

So we need both /lib and /lib64 to be present, even though the canonical linker
path uses /lib.