hermitkernel-target: Set OS to "none"

For our kernel targets, we should not set OS, as the kernel runs bare
metal without a circular dependency on std.

This also prepares us for unifying with
#89062. This patch requires
libhermit-rs to change a `cfg`s from `target_os = "hermit"` to `target_os
= "none"`.

I tested this patch locally.

compiler/rustc_target/src/spec/hermit_kernel_base.rs

@@ -8,7 +8,6 @@ pub fn opts() -> TargetOptions {
     );
 
     TargetOptions {
-        os: "hermit".to_string(),
         linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
         disable_redzone: true,
         linker: Some("rust-lld".to_owned()),