feat: support cross-compiling to aarch64-linux-android

[eval-exec]

What problem does this PR solve?

CKB has not been buildable for aarch64-linux-android. Cross-compiling out of the box fails on:

• daemonize-me (referenced by ckb-bin) does not support target_os = "android" — its FFIPasswd struct is gated to linux/macos/freebsd/openbsd/netbsd only.
• ckb-librocksdb-sys's build_detect_platform script unconditionally adds x86 SSE/AVX/BMI/PCLMUL flags when the probing compiler accepts them, which breaks the NDK aarch64 clang during the C++ build.
• bindgen for rocksdb cannot find Android system headers without explicit sysroot/clang configuration.
• NDK 23+ no longer ships libgcc, but rustc still passes -lgcc to the linker when targeting *-linux-android.

Problem Summary: provide a reproducible cross-compilation environment (Nix flake, NixOS-friendly) and the small source patches needed to actually build ckb for aarch64-linux-android.

What is changed and how it works?

What's Changed:

• flake.nix (new): devShell wiring rust-overlay (rustc 1.92.0 + aarch64-linux-android target) and pkgsCross.aarch64-android-prebuilt (Android NDK 27).
  • HOST_CC/CXX/AR/LD/RANLIB point at host gcc/g++/ar/ld/ranlib so cargo build scripts that compile and link on the host work correctly.
  • CC/CXX/AR/RANLIB/STRIP and CARGO_TARGET_AARCH64_LINUX_ANDROID_* point at the NDK toolchain so the target compiler is used for cross-compilation (and so build_detect_platform's SSE probes correctly fail on aarch64, skipping the x86 flags).
  • LIBCLANG_PATH and BINDGEN_EXTRA_CLANG_ARGS use the NDK's bundled clang-18 with the unwrapped sysroot so bindgen finds Android headers (uint32_t, etc.).
  • TARGET_OS=OS_ANDROID_CROSSCOMPILE and TARGET_ARCHITECTURE=aarch64 hint rocksdb's detect script.
  • A stub libgcc.a containing INPUT(-lunwind) is generated and added to the Android linker search path via CARGO_TARGET_AARCH64_LINUX_ANDROID_RUSTFLAGS to satisfy rustc's hardcoded -lgcc.
• .cargo/config.toml: add empty [target.aarch64-linux-android] stanza; linker comes from the env vars set by the devShell.
• .gitignore: ignore .android-libgcc-stub/.
• Source cfg changes: extend cfg(not(target_os = "windows")) to also exclude target_os = "android" for daemonization-related code, in:
  • ckb-bin/Cargo.toml, ckb-bin/src/{lib.rs,setup.rs,cli.rs,subcommand/mod.rs}
  • util/app-config/src/args.rs (the daemon field of RunArgs)

Build command:

nix develop --command cargo build --release --target aarch64-linux-android --bin ckb

Produces target/aarch64-linux-android/release/ckb:

ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV),
dynamically linked, interpreter /system/bin/linker64

Related changes

• No companion PRs.
• No cherry-pick to release branches needed.

Check List

Tests

• Manual test: built --release --bin ckb for aarch64-linux-android from a NixOS x86_64 host using nix develop; verified the resulting ELF is ARM aarch64 and dynamically linked against Android's /system/bin/linker64.

Side effects

• No behavior change for existing supported targets: changes are limited to additional cfg exclusions for target_os = "android" (which was previously unbuildable) and a new flake.nix/.cargo/config.toml stanza that only affects the new target.