shared/base-filesystem: add define for riscv64

shared/base-filesystem: add define for riscv64

https://wiki.debian.org/ArchitectureSpecificsMemo shows the triplet, but no the
linker paths. I used the linker path from Fedora.

$ ls -l /lib /lib64
lrwxrwxrwx. 1 root root 7 Aug 13  2020 /lib -> usr/lib
lrwxrwxrwx. 1 root root 9 Aug 13  2020 /lib64 -> usr/lib64
$ ldd /bin/sh|grep ld
	/lib/ld-linux-riscv64-lp64d.so.1 (0x0000003fb8185000)
$ ls -l /lib/ld-linux-riscv64-lp64d.so.1
lrwxrwxrwx 1 root root 19 Aug  4 19:28 /lib/ld-linux-riscv64-lp64d.so.1 -> ../lib64/ld-2.32.so

$ uname -r
5.10.6+

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