build stub crt libs and avoid --sysroot

cross-build-rtlibs.sh

@@ -2,15 +2,24 @@
 # build compiler-rt, libcxx, libcxxabi, and libunwind with our patched clang
 # see https://llvm.org/docs/HowToCrossCompileBuiltinsOnArm.html
 set -x
+
+# build stub crt object files to prevent wrong-arch ones from being selected
+fake_crt_dir=$(pwd)/fake_crt
+mkdir -p $fake_crt_dir
+for crtobj in crtn.o crti.o Scrt1.o; do
+	aarch64-linux-gnu-gcc -c -x c /dev/null -o $fake_crt_dir/$crtobj
+done
+
+# build runtime libs
 mkdir -p build-rtlibs
 cd build-rtlibs
-ls -l /usr/lib/gcc/aarch64-linux-gnu
-find /usr/lib | fgrep crtbeginS.o
+
+# find existing aarch64 crt for building test programs
 crt_candidates="$(ls -d /usr/lib/gcc/aarch64-linux-gnu/*)
 $(ls -d /usr/lib/gcc-cross/aarch64-linux-gnu/*)"
 crt_dir=$(echo $crt_candidates | sort -V | tail -n1)
 
-cross_flags="-B$crt_dir --sysroot=/usr/aarch64-linux-gnu --gcc-install-dir=$crt_dir -isystem /usr/aarch64-linux-gnu/include -march=armv8+memtag -ffixed-x18"
+cross_flags="-B$fake_crt_dir -B$crt_dir --gcc-toolchain=/usr --gcc-triple=aarch64-linux-gnu -v -isystem /usr/aarch64-linux-gnu/include -march=armv8+memtag -ffixed-x18"
 #--sysroot=/usr/aarch64-linux-gnu/ --gcc-install-dir=/usr/lib/gcc/aarch64-linux-gnu/14.1.0
 export LDFLAGS="-L/usr/aarch64-linux-gnu/lib"
 cmake -GNinja -DLLVM_TARGETS_TO_BUILD="AArch64" -DLLVM_DEFAULT_TARGET_TRIPLE="aarch64-linux-gnu" \