Forbid rustc from using -f{function,data}-sections (fixes #20 panic)

Makefile

@@ -510,11 +510,9 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_RUSTCFLAGS :=
 # TODO: a simple way to update `Cargo.lock` when we add a new driver
-# TODO: another option is using explicit target specs, e.g.
-# `--target=$(srctree)/arch/$(SRCARCH)/rust-target-spec.json`
 KBUILD_CARGOFLAGS := $(CARGO_VERBOSE) --locked \
 		    -Z build-std=core,alloc -Z unstable-options \
-		    --out-dir=out --target=x86_64-linux-kernel
+		    --out-dir=out --target=$(PWD)/$(srctree)/arch/$(SRCARCH)/rust/target.json
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=
 KBUILD_RUSTCFLAGS_KERNEL :=

arch/x86/rust/target.json

@@ -0,0 +1,34 @@
+{
+  "arch": "x86_64",
+  "code-model": "kernel",
+  "cpu": "x86-64",
+  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
+  "disable-redzone": true,
+  "eliminate-frame-pointer": false,
+  "env": "gnu",
+  "features": "-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-3dnow,-3dnowa,-avx,-avx2,+soft-float",
+  "function-sections": false,
+  "is-builtin": true,
+  "linker-flavor": "gcc",
+  "linker-is-gnu": true,
+  "llvm-target": "x86_64-elf",
+  "max-atomic-width": 64,
+  "needs-plt": true,
+  "os": "none",
+  "panic-strategy": "abort",
+  "position-independent-executables": true,
+  "pre-link-args": {
+    "gcc": [
+      "-Wl,--as-needed",
+      "-Wl,-z,noexecstack",
+      "-m64"
+    ]
+  },
+  "relocation-model": "static",
+  "relro-level": "full",
+  "stack-probes": true,
+  "target-c-int-width": "32",
+  "target-endian": "little",
+  "target-pointer-width": "64",
+  "vendor": "unknown"
+}

rust/kernel/build.rs

@@ -96,7 +96,8 @@ fn main() {
 
     let kernel_args = prepare_cflags(&cflags, &kernel_dir);
 
-    let target = env::var("TARGET").unwrap();
+    // TODO: pass the proper triple to bindgen
+    let target = "x86_64-linux-kernel";
 
     let mut builder = bindgen::Builder::default()
         .use_core()