Change built-in kernel targets to be os = none throughout

Whether for Rust's own `target_os`, LLVM's triples, or GNU config's, the
OS-related have fields have been for code running *on* that OS, not code
that is *part* of the OS.

The difference is huge, as syscall interfaces are nothing like
freestanding interfaces. Kernels are (hypervisors and other more exotic
situations aside) freestanding programs that use the interfaces provided
by the hardware. It's *those* interfaces, the ones external to the
program being built and its software dependencies, that are the content
of the target.

For the Linux Kernel in particular, `target_env: "gnu"` is removed for
the same reason: that `-gnu` refers to glibc or GNU/linux, neither of
which applies to the kernel itself.

Relates to #74247

Thanks @ojeda for catching some things.

compiler/rustc_target/src/spec/mod.rs

@@ -679,7 +679,7 @@ supported_targets! {
     ("thumbv7neon-linux-androideabi", thumbv7neon_linux_androideabi),
     ("aarch64-linux-android", aarch64_linux_android),
 
-    ("x86_64-linux-kernel", x86_64_linux_kernel),
+    ("x86_64-unknown-none-linuxkernel", x86_64_unknown_none_linuxkernel),
 
     ("aarch64-unknown-freebsd", aarch64_unknown_freebsd),
     ("armv6-unknown-freebsd", armv6_unknown_freebsd),
@@ -777,7 +777,8 @@ supported_targets! {
 
     ("aarch64-unknown-hermit", aarch64_unknown_hermit),
     ("x86_64-unknown-hermit", x86_64_unknown_hermit),
-    ("x86_64-unknown-hermit-kernel", x86_64_unknown_hermit_kernel),
+
+    ("x86_64-unknown-none-hermitkernel", x86_64_unknown_none_hermitkernel),
 
     ("riscv32i-unknown-none-elf", riscv32i_unknown_none_elf),
     ("riscv32imc-unknown-none-elf", riscv32imc_unknown_none_elf),

compiler/rustc_target/src/spec/x86_64_unknown_hermit_kernel.rs → compiler/rustc_target/src/spec/x86_64_unknown_none_hermitkernel.rs

@@ -10,7 +10,7 @@ pub fn target() -> Target {
     base.stack_probes = StackProbeType::InlineOrCall { min_llvm_version_for_inline: (11, 0, 1) };
 
     Target {
-        llvm_target: "x86_64-unknown-hermit".to_string(),
+        llvm_target: "x86_64-unknown-none-elf".to_string(),
         pointer_width: 64,
         data_layout: "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
             .to_string(),

compiler/rustc_target/src/spec/x86_64_linux_kernel.rs → compiler/rustc_target/src/spec/x86_64_unknown_none_linuxkernel.rs

@@ -14,8 +14,11 @@ pub fn target() -> Target {
     base.pre_link_args.get_mut(&LinkerFlavor::Gcc).unwrap().push("-m64".to_string());
 
     Target {
-        // FIXME: Some dispute, the linux-on-clang folks think this should use "Linux"
-        llvm_target: "x86_64-elf".to_string(),
+        // FIXME: Some dispute, the linux-on-clang folks think this should use
+        // "Linux". We disagree because running *on* Linux is nothing like
+        // running *as" linux, and historically the "os" component as has always
+        // been used to mean the "on" part.
+        llvm_target: "x86_64-unknown-none-elf".to_string(),
         pointer_width: 64,
         data_layout: "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
             .to_string(),

library/unwind/src/lib.rs

@@ -15,7 +15,7 @@ cfg_if::cfg_if! {
         target_os = "none",
     ))] {
         // These "unix" family members do not have unwinder.
-        // Note this also matches x86_64-linux-kernel.
+        // Note this also matches x86_64-unknown-none-linuxkernel.
     } else if #[cfg(any(
         unix,
         windows,

src/doc/rustc/src/platform-support.md

@@ -217,13 +217,13 @@ target | std | host | notes
 `thumbv4t-none-eabi` | * |  | ARMv4T T32
 `x86_64-apple-ios-macabi` | ✓ |  | Apple Catalyst on x86_64
 `x86_64-apple-tvos` | * | | x86 64-bit tvOS
-`x86_64-linux-kernel` | * |  | Linux kernel modules
+`x86_64-unknown-none-linuxkernel` | * |  | Linux kernel modules
 `x86_64-sun-solaris` | ? |  | Deprecated target for 64-bit Solaris 10/11, illumos
 `x86_64-pc-windows-msvc` | ✓ |  | 64-bit Windows XP support
 `x86_64-unknown-dragonfly` | ✓ | ✓ | 64-bit DragonFlyBSD
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku
 `x86_64-unknown-hermit` | ? |  |
-`x86_64-unknown-hermit-kernel` | ? |  | HermitCore kernel
+`x86_64-unknown-none-hermitkernel` | ? |  | HermitCore kernel
 `x86_64-unknown-l4re-uclibc` | ? |  |
 `x86_64-unknown-openbsd` | ✓ | ✓ | 64-bit OpenBSD
 `x86_64-unknown-uefi` | ? |  |