Update ARM64 CI to Ubuntu 24.04 and fix bootstrap script defaults

- Update ARM64 runners from ubuntu-22.04-arm to ubuntu-24.04-arm
- Update LLVM apt repository from jammy to noble for Ubuntu 24.04
- Remove clang default from bootstrap-cmake-linux.sh; now uses
  system default compiler unless CC/CXX are explicitly set

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/linux_ci.yml

@@ -118,7 +118,7 @@ jobs:
 
   build-arm64-gcc:
 
-    runs-on: ubuntu-22.04-arm
+    runs-on: ubuntu-24.04-arm
 
     steps:
     - uses: actions/checkout@v2
@@ -136,7 +136,7 @@ jobs:
 
   build-arm64-clang:
 
-    runs-on: ubuntu-22.04-arm
+    runs-on: ubuntu-24.04-arm
 
     steps:
     - uses: actions/checkout@v2
@@ -149,7 +149,7 @@ jobs:
     - name: install clang-21
       run: |
         wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo tee /etc/apt/trusted.gpg.d/apt.llvm.org.asc
-        sudo add-apt-repository -y "deb http://apt.llvm.org/jammy/ llvm-toolchain-jammy-21 main"
+        sudo add-apt-repository -y "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-21 main"
         sudo apt-get update
         sudo apt-get install -y clang-21
     - name: configure

scripts/bootstrap-cmake-linux.sh

@@ -5,14 +5,17 @@ builddir=${curdir}/build
 rm -rf ${builddir}
 mkdir ${builddir}
 
-# Use environment CC/CXX if set, otherwise default to clang
-: ${CC:=clang}
-: ${CXX:=clang++}
-
 # with lld linker
 #  -DCMAKE_TOOLCHAIN_FILE=cmake/lld-linux.toolchain.cmake
 
-cd ${builddir} && CC=${CC} CXX=${CXX} cmake \
-  -DCMAKE_VERBOSE_MAKEFILE=1 \
-  ..
+# Use CC/CXX environment variables if set, otherwise let CMake detect
+if [ -n "${CC}" ] && [ -n "${CXX}" ]; then
+  cd ${builddir} && CC=${CC} CXX=${CXX} cmake \
+    -DCMAKE_VERBOSE_MAKEFILE=1 \
+    ..
+else
+  cd ${builddir} && cmake \
+    -DCMAKE_VERBOSE_MAKEFILE=1 \
+    ..
+fi