ci: standardize custom libc++ usage in MSAN jobs

Use `-isystem` & `-nostd*` flags, which is the preferred way to use a
custom libc++ (ours is libc++ build with MSAN) with Clang, as opposed to
our current ad-hoc flags.

See: https://releases.llvm.org/16.0.0/projects/libcxx/docs/UsingLibcxx.html#using-a-custom-built-libc
for more info.

Author: fanquake

ci/test/00_setup_env_native_fuzz_with_msan.sh

@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 export CI_IMAGE_NAME_TAG="ubuntu:23.04" # Version 23.04 will reach EOL in Jan 2024, and can be replaced by "ubuntu:24.04" (or anything else that ships the wanted clang version).
 LIBCXX_DIR="${BASE_SCRATCH_DIR}/msan/build/"
 export MSAN_FLAGS="-fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -g -O1 -fno-optimize-sibling-calls"
-LIBCXX_FLAGS="-nostdinc++ -stdlib=libc++ -L${LIBCXX_DIR}lib -lc++abi -I${LIBCXX_DIR}include -I${LIBCXX_DIR}include/c++/v1 -lpthread -Wl,-rpath,${LIBCXX_DIR}lib -Wno-unused-command-line-argument"
+LIBCXX_FLAGS="-nostdinc++ -nostdlib++ -isystem ${LIBCXX_DIR}include/c++/v1 -L${LIBCXX_DIR}lib -Wl,-rpath,${LIBCXX_DIR}lib -lc++ -lc++abi -lpthread -Wno-unused-command-line-argument"
 export MSAN_AND_LIBCXX_FLAGS="${MSAN_FLAGS} ${LIBCXX_FLAGS}"
 
 export CONTAINER_NAME="ci_native_fuzz_msan"

ci/test/00_setup_env_native_msan.sh

@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 export CI_IMAGE_NAME_TAG="ubuntu:23.04" # Version 23.04 will reach EOL in Jan 2024, and can be replaced by "ubuntu:24.04" (or anything else that ships the wanted clang version).
 LIBCXX_DIR="${BASE_SCRATCH_DIR}/msan/build/"
 export MSAN_FLAGS="-fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -g -O1 -fno-optimize-sibling-calls"
-LIBCXX_FLAGS="-nostdinc++ -stdlib=libc++ -L${LIBCXX_DIR}lib -lc++abi -I${LIBCXX_DIR}include -I${LIBCXX_DIR}include/c++/v1 -lpthread -Wl,-rpath,${LIBCXX_DIR}lib -Wno-unused-command-line-argument"
+LIBCXX_FLAGS="-nostdinc++ -nostdlib++ -isystem ${LIBCXX_DIR}include/c++/v1 -L${LIBCXX_DIR}lib -Wl,-rpath,${LIBCXX_DIR}lib -lc++ -lc++abi -lpthread -Wno-unused-command-line-argument"
 export MSAN_AND_LIBCXX_FLAGS="${MSAN_FLAGS} ${LIBCXX_FLAGS}"
 
 export CONTAINER_NAME="ci_native_msan"