Speed up kernel CI: ccache depend mode, drop pahole deps, no module compression

- ccache: enable depend mode (skip the preprocessor on ~75k compiler
  calls) and add BASEDIR/NOHASHDIR/SLOPPINESS as insurance against false
  misses from build-path or build-macro churn. ccache -s already shows
  99.99% direct hits, so this trims per-call overhead, not a broken
  cache. MAXSIZE stays 8G (only ~1G used).
- deps: drop dwarves + libdw-dev. pahole only runs with
  CONFIG_DEBUG_INFO_BTF, but the build forces DEBUG_INFO_NONE, so they
  were never used.
- config: disable MODULE_COMPRESS/MODULE_DECOMPRESS for both arches.
  arm64 alone shipped zstd module compression, running a zstd pass over
  ~8000 .ko every build for no gain (the .deb is already compressed by
  KDEB_COMPRESS); x86_64 never did. Keeps the two arches in sync.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -129,12 +129,15 @@ jobs:
         if: env.BUILD_NEEDED == 'true'
         run: |
           sudo apt-get update
+          # dwarves/libdw-dev (pahole) are intentionally absent: the build sets
+          # CONFIG_DEBUG_INFO_NONE=y and DEBUG_INFO_BTF=n (see prepare-kernel-config.sh),
+          # so pahole never runs. Re-add them if BTF is ever enabled.
           sudo apt-get install -y \
             git build-essential \
             libncurses-dev libssl-dev libelf-dev \
             bison bc flex rsync debhelper \
-            dpkg-dev fakeroot kmod cpio dwarves \
-            libdw-dev lz4 zstd xz-utils curl jq
+            dpkg-dev fakeroot kmod cpio \
+            lz4 zstd xz-utils curl jq
 
       - name: 安装 ARM64 交叉编译工具链
         if: env.BUILD_NEEDED == 'true' && matrix.arch == 'arm64'
@@ -159,6 +162,19 @@ jobs:
             echo "CCACHE_DIR=$HOME/.ccache"
             echo "CCACHE_MAXSIZE=8G"
             echo "CCACHE_COMPRESS=1"
+            # depend mode: hash the source + its -MD/-MMD header list instead of
+            # running the preprocessor on every one of ~75k compiler calls. The
+            # kernel always emits dependency files, so this is safe and trims the
+            # per-call overhead on both hits and misses.
+            echo "CCACHE_DEPEND=1"
+            # Insurance against false misses if the build path or build-time macros
+            # ever vary between runs (the source is re-cloned fresh each time).
+            # BASEDIR must be an ancestor of the build CWD (./kernel/linux).
+            # With the path already stable these are near no-ops, but they cost
+            # nothing and guard against a path/macro change silently going cold.
+            echo "CCACHE_BASEDIR=$GITHUB_WORKSPACE/kernel"
+            echo "CCACHE_NOHASHDIR=1"
+            echo "CCACHE_SLOPPINESS=time_macros,file_macros,locale,pch_defines"
           } >> "$GITHUB_ENV"
 
       - name: 恢复 ccache 缓存