CUBELinux.0.1: name the release, and build with this box's rustc

The tree is Linux 6.19.3 with the changes this machine's toolchain needs, and
nothing else. No CUBE code yet — this is the base the coordinate interface will
be built on, so it starts from a known-good bootable kernel.

Naming:
- VERSION/PATCHLEVEL/SUBLEVEL stay 6.19.3 (visible in `make kernelversion`),
  while the release string setlocalversion composes is CUBELinux.0.1, so
  `uname -r` and /lib/modules report this product rather than a Linux point
  release. The SCM suffix still applies: a dirty tree says so.

rustc compatibility (rustc 1.100.0-nightly, clang 19.1.7):
- scripts/generate_rust_target.rs emitted `"rustc-abi": "x86-softfloat"`, which
  this rustc rejects; it is `softfloat` now.
- rust/Makefile's cmd_rustc_library did not pass -Zunstable-options, so the
  custom target spec would not load at all.
- the generated bindings declare `strlen` with the kernel target's `c_char`
  (u8, from -funsigned-char) while rustc expects `*const i8`; the newer
  suspicious_runtime_symbol_definitions lint fires on that and -D warnings makes
  it fatal. Scoped to bindings.o and uapi.o, not to handwritten code.
- three `'static` bounds the abstractions now need (irq handlers).
- str.rs imported alloc::flags::* which prelude::* already provides, and
  `#![feature(used_with_arg)]` is stale now that the feature is stable; both are
  unused-feature/unused-import errors under -D warnings.

Gates: bzImage builds (14,697,472 bytes) with CONFIG_RUST=y, virtio-blk and a
serial console built in. Boot test in QEMU is next.

rust/kernel/lib.rs

@@ -40,8 +40,8 @@
 // Expected to become stable.
 #![feature(arbitrary_self_types)]
 //
-// To be determined.
-#![feature(used_with_arg)]
+// `used_with_arg` was stabilised; the attribute below is used without a gate now, and a
+// stale `#![feature]` for it is an error under `-D warnings` on this rustc.
 //
 // `feature(derive_coerce_pointee)` is expected to become stable. Before Rust
 // 1.84.0, it did not exist, so enable the predecessor features.