tools: clean unwanted ELF flags from binaries

Fixes warnings on old Android, such as:

linker  : /data/data/com.wireguard.android/code_cache/bin/wg-quick: unsupported flags DT_FLAGS_1=0x8000001
linker  : /data/data/com.wireguard.android/code_cache/bin/wg: unsupported flags DT_FLAGS_1=0x8000001

Reported-by: L.W.Reek <syphyr@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: zx2c4

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "tunnel/tools/wireguard-tools"]
 	path = tunnel/tools/wireguard-tools
 	url = https://git.zx2c4.com/wireguard-tools
+[submodule "tunnel/tools/elf-cleaner"]
+	path = tunnel/tools/elf-cleaner
+	url = https://github.com/termux/termux-elf-cleaner

tunnel/tools/CMakeLists.txt

@@ -28,3 +28,16 @@ add_custom_target(libwg-go.so WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/lib
     DESTDIR=${CMAKE_LIBRARY_OUTPUT_DIRECTORY}
     BUILDDIR=${CMAKE_LIBRARY_OUTPUT_DIRECTORY}/../generated-src
 )
+
+# Strip unwanted ELF sections to prevent DT_FLAGS_1 warnings on old Android versions
+file(GLOB ELF_CLEANER_SOURCES elf-cleaner/*.c elf-cleaner/*.cpp)
+add_custom_target(elf-cleaner COMMENT "Building elf-cleaner" VERBATIM COMMAND "${CMAKE_C_COMPILER}"
+        -O2 -DPACKAGE_NAME="elf-cleaner" -DPACKAGE_VERSION="" -DCOPYRIGHT=""
+        -o "${CMAKE_CURRENT_BINARY_DIR}/elf-cleaner" ${ELF_CLEANER_SOURCES}
+)
+add_custom_command(TARGET libwg.so POST_BUILD VERBATIM COMMAND "${CMAKE_CURRENT_BINARY_DIR}/elf-cleaner"
+        --api-level "${ANDROID_NATIVE_API_LEVEL}" "$<TARGET_FILE:libwg.so>")
+add_dependencies(libwg.so elf-cleaner)
+add_custom_command(TARGET libwg-quick.so POST_BUILD VERBATIM COMMAND "${CMAKE_CURRENT_BINARY_DIR}/elf-cleaner"
+        --api-level "${ANDROID_NATIVE_API_LEVEL}" "$<TARGET_FILE:libwg-quick.so>")
+add_dependencies(libwg-quick.so elf-cleaner)