Add a toolchain for toolchain building

kotlin-native/tools/toolchain_builder/Dockerfile

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+# We might want to switch to alpine, but it is not stable enough yet.
+FROM ubuntu:20.04
+
+ENV TZ=Europe/Moscow
+RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
+
+# Install crosstool-ng deps.
+RUN apt-get update
+RUN apt-get install -y curl gcc git g++ gperf bison flex texinfo help2man make libncurses5-dev \
+    python3-dev autoconf automake libtool libtool-bin gawk wget bzip2 xz-utils unzip \
+    patch libstdc++6 rsync
+
+# Put a fix for strip.
+COPY patches/github_pull_1244.patch .
+# Install crosstool-ng.
+RUN git clone --branch crosstool-ng-1.24.0 --depth 1 https://github.com/crosstool-ng/crosstool-ng.git && \
+    cd crosstool-ng && \
+    git checkout b2151f1dba2b20c310adfe7198e461ec4469172b && \
+    git apply ../github_pull_1244.patch && \
+    ./bootstrap && ./configure && make && make install && \
+    cd .. && rm -rf crosstool-ng
+
+# Create a user.
+ARG USERNAME=ct
+RUN groupadd -g 1000 $USERNAME
+RUN useradd -r -u 1000 --create-home -g $USERNAME $USERNAME
+USER $USERNAME
+WORKDIR /home/$USERNAME
+
+# Download zlib sources.
+RUN curl -LO https://zlib.net/zlib-1.2.11.tar.gz && \
+    tar -xf zlib-1.2.11.tar.gz && \
+    rm zlib-1.2.11.tar.gz
+
+# Save crosstool-ng config files.
+COPY toolchains toolchains
+
+# Used by crosstool-ng.
+RUN mkdir src
+
+ENV TARGET=x86_64-unknown-linux-gnu
+ENV VERSION=gcc-8.3.0-glibc-2.19-kernel-4.9
+
+# Add entry point.
+COPY build_toolchain.sh .
+ENTRYPOINT "/bin/bash" "build_toolchain.sh" ${TARGET} ${VERSION}