fix(build): detect native ARM64 host and skip cross-compiler requirement

build-kernel-arm64.sh and build-kernel-rpi.sh both insisted on
aarch64-linux-gnu-gcc (the cross-compiler from x86), which fails on a native
ARM64 build host like the Odroid runner. Detect uname -m and use the host's
gcc with an empty CROSS_COMPILE on aarch64 hosts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/scripts/build-kernel-rpi.sh

@@ -35,11 +35,24 @@ if [ -f "$CUSTOM_IMAGE" ] && [ -d "$CUSTOM_MODULES" ]; then
     exit 0
 fi
 
-# --- Verify cross-compiler ---
-if ! command -v aarch64-linux-gnu-gcc >/dev/null 2>&1; then
-    echo "ERROR: aarch64-linux-gnu-gcc not found"
-    echo "Install: apt-get install gcc-aarch64-linux-gnu"
-    exit 1
+# --- Toolchain selection: native on arm64 hosts, cross-compile elsewhere ---
+HOST_ARCH="$(uname -m)"
+if [ "$HOST_ARCH" = "aarch64" ] || [ "$HOST_ARCH" = "arm64" ]; then
+    if ! command -v gcc >/dev/null 2>&1; then
+        echo "ERROR: gcc not found"
+        echo "Install: apt-get install build-essential"
+        exit 1
+    fi
+    CROSS_COMPILE=""
+    echo "==> Native ARM64 build (host arch: $HOST_ARCH)"
+else
+    if ! command -v aarch64-linux-gnu-gcc >/dev/null 2>&1; then
+        echo "ERROR: aarch64-linux-gnu-gcc not found"
+        echo "Install: apt-get install gcc-aarch64-linux-gnu"
+        exit 1
+    fi
+    CROSS_COMPILE="aarch64-linux-gnu-"
+    echo "==> Cross-building RPi kernel from $HOST_ARCH"
 fi
 
 echo "==> Building RPi kernel (raspberrypi/linux)..."
@@ -65,7 +78,7 @@ cd "$KERNEL_BUILD_DIR"
 
 # --- Apply base config (Pi 4 = bcm2711) ---
 echo "==> Applying bcm2711_defconfig..."
-make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcm2711_defconfig
+make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE" bcm2711_defconfig
 
 # --- Apply container config overrides ---
 CONFIG_FRAGMENT="$PROJECT_ROOT/build/config/kernel-container.fragment"
@@ -98,7 +111,7 @@ if [ -f "$CONFIG_FRAGMENT" ]; then
 fi
 
 # Resolve dependencies
-make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- olddefconfig
+make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE" olddefconfig
 
 # --- Build kernel + modules + DTBs ---
 NPROC=$(nproc 2>/dev/null || echo 4)
@@ -106,7 +119,7 @@ echo ""
 echo "==> Building RPi kernel (${NPROC} parallel jobs)..."
 echo "    This may take 20-30 minutes..."
 
-make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j"$NPROC" Image modules dtbs 2>&1
+make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE" -j"$NPROC" Image modules dtbs 2>&1
 
 echo "==> RPi kernel build complete"
 
@@ -117,7 +130,7 @@ cp arch/arm64/boot/Image "$CUSTOM_IMAGE"
 echo "==> Installing modules (stripped)..."
 rm -rf "$CUSTOM_MODULES"
 mkdir -p "$CUSTOM_MODULES"
-make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
+make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE" \
     INSTALL_MOD_STRIP=1 modules_install INSTALL_MOD_PATH="$CUSTOM_MODULES"
 
 # Remove build/source symlinks