add script to build compiler-rt

cross-build-compiler-rt.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# see https://llvm.org/docs/HowToCrossCompileBuiltinsOnArm.html
+mkdir -p build-compiler-rt
+cd build-compiler-rt
+cross_flags="--gcc-toolchain=/usr --sysroot=/usr/aarch64-linux-gnu -march=armv8.5-a+memtag -ffixed-x18"
+export LDFLAGS="-L/usr/aarch64-linux-gnu/lib"
+cmake -GNinja -DLLVM_TARGETS_TO_BUILD="AArch64" \
+    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+    -DLLVM_DEFAULT_TARGET_TRIPLE="aarch64-linux-gnu" -DCMAKE_C_FLAGS="$cross_flags --target=aarch64-linux-gnu" -DCMAKE_CXX_FLAGS="$cross_flags --target=aarch64-linux-gnu" -DCMAKE_C_COMPILER="$(pwd)/../build/bin/clang" -DCMAKE_CXX_COMPILER="$(pwd)/../build/bin/clang" -DCMAKE_BUILD_WITH_INSTALL_RPATH=true -DCOMPILER_RT_BUILD_SANITIZERS=OFF -DCOMPILER_RT_BUILD_MEMPROF=OFF -DCOMPILER_RT_BUILD_ORC=OFF -DCOMPILER_RT_BUILD_XRAY=OFF -DCOMPILER_RT_BUILD_LIBFUZZER=OFF -DCOMPILER_RT_DEFAULT_TARGET_TRIPLE="aarch64-linux-gnu" ../compiler-rt
+ninja
+cd lib/linux
+# rename CRT files to expected filenames
+cp -a clang_rt.crtend-aarch64.o crtendS.o
+cp -a clang_rt.crtbegin-aarch64.o crtbeginS.o