The arm path hardcoded ARCH=x86_64 and the x86_64 type2 runtime, so a
`build_release.sh arm` run produced a broken AppImage (x86_64 runtime wrapping
an aarch64 binary). Derive ARCH and the runtime file from the target arch, and
have toolchain.sh fetch runtime-aarch64 alongside runtime-x86_64. This restores
GRIM's linux-arm AppImage release type; verified the output is an ARM aarch64
AppImage.
Add scripts/toolchain.sh: fetches GRIM's canonical build toolchains (custom
NDK r29, zig, appimagetool + type2 runtime, and optionally the Android SDK /
gradle / osxcross) into a gitignored .toolchains/ and writes env.sh.
linux/build_release.sh and scripts/android.sh now source .toolchains/env.sh,
preferring the DEV toolchains and falling back to system installs:
- Android links against the custom NDK r29 (rebuilt LLVM), producing
16 KB page-aligned .so libraries — required by the Play Store.
- The Linux AppImage cross-builds with the DEV zig + appimagetool; bindgen is
pointed at the host kernel headers so v4l2-sys finds linux/videodev2.h under
zig's glibc-2.17 sysroot.
Also fix the stale .gitignore AppRun entry (Grim.AppDir -> Goblin.AppDir).
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