shared/base-filesystem: add define for arm

$ ls -l /lib /lib64
ls: cannot access '/lib64': No such file or directory
lrwxrwxrwx. 1 root root 7 Jan 26  2021 /lib -> usr/lib
$ ldd /bin/sh|grep ld
	/lib/ld-linux-armhf.so.3 (0xb6f80000)

src/shared/base-filesystem.c

@@ -55,6 +55,8 @@ static const BaseFilesystem table[] = {
 #elif defined(__alpha__)
 #elif defined(__arc__) || defined(__tilegx__)
 #elif defined(__arm__)
+        /* No /lib64 on arm. The linker is /lib/ld-linux-armhf.so.3. */
+#  define KNOW_LIB64_DIRS 1
 #elif defined(__i386__) || defined(__x86_64__)
         { "lib64",    0, "usr/lib/x86_64-linux-gnu\0"
                          "usr/lib64\0",                "ld-linux-x86-64.so.2" },