rust-target-config: fix nativesdk-libstd-rs build with baremetal

rust-target-config: fix nativesdk-libstd-rs build with baremetal

If TCLIBC='baremetal' is set in local.conf, nativesdk-libstd-rs build fails
with:

| error[E0412]: cannot find type `c_char` in the crate root
|   --> /usr/src/debug/libstd-rs/1.75.0/rustc-1.75.0-src/vendor/libc/src/unix/mod.rs:56:29
|    |
| 6  | pub type c_schar = i8;
|    | ---------------------- similarly named type alias `c_schar` defined here
| ...
| 56 |         pub gr_name: *mut ::c_char,
|    |                             ^^^^^^

This happens because rust_gen_target() sets os="none" when TCLIBC is
'baremetal' - even for nativesdk targets. However, nativesdk packages are
built against glibc, so the correct 'os' value should be "linux".

Fix this by setting the os field based on {TARGET,HOST,BUILD}_OS variables,
as it is already done in rust_base_triple(), instead of relying on TCLIBC.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>