update toolchain to nightly-2026-06-01 and fix build

The newer nightly changed how custom target specs are handled:

- .cargo/config.toml: enable the `json-target-spec` unstable flag,
  now required to use `.json` target files
- aarch64-eduos.json: add `rustc-abi: "softfloat"`, now needed
  alongside `abi` for bare-metal aarch64 softfloat targets
- src/lib.rs: drop the stabilized `allocator_api` feature gate

Both targets build and boot in QEMU again:
  cargo run
  cargo run --target aarch64-eduos.json

Author: stlankes

.cargo/config.toml

@@ -1,6 +1,7 @@
 [unstable]
 build-std = ["core", "alloc"]
 build-std-features = ["compiler-builtins-mem"]
+json-target-spec = true
 
 [build]
 target = "x86_64-eduos.json"

aarch64-eduos.json

@@ -0,0 +1,17 @@
+{
+    "llvm-target": "aarch64-unknown-none",
+    "target-endian": "little",
+    "target-pointer-width": 64,
+    "target-c-int-width": 32,
+    "os": "none",
+    "arch": "aarch64",
+    "abi": "softfloat",
+    "rustc-abi": "softfloat",
+    "features": "+v8a,+strict-align,-neon,-fp-armv8",
+    "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128-Fn32",
+    "disable-redzone": true,
+    "executables": true,
+    "linker-flavor": "ld.lld",
+    "linker": "rust-lld",
+    "panic-strategy": "abort"
+}

rust-toolchain.toml

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "nightly-2025-10-01"
+channel = "nightly-2026-06-01"
 components = [
     "clippy",
     "rustfmt",