Add support for sparc64 in uucore to avoid the following build failure
with nushell:

error[E0308]: mismatched types
   --> /home/autobuild/autobuild/instance-7/output-1/build/nushell-0.85.0/VENDOR/uucore/src/lib/features/fs.rs:121:16
    |
111 |     pub fn number_of_links(&self) -> u64 {
    |                                      --- expected `u64` because of return type
...
121 |         return self.0.st_nlink;
    |                ^^^^^^^^^^^^^^^ expected `u64`, found `u32`
    |
help: you can convert a `u32` to a `u64`
    |
121 |         return self.0.st_nlink.into();
    |                               +++++++

For more information about this error, try `rustc --explain E0308`.
error: could not compile `uucore` (lib) due to previous error

Fixes:
 - http://autobuild.buildroot.org/results/f9f0287a8e39c65895014ca513ed25071f020add

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>