add php aarch64 linux emulated tests

kokoro/linux/aarch64/php_build_and_run_tests_with_qemu_aarch64.sh

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+#!/bin/bash
+
+set -ex
+
+# Install composer
+curl -sS https://getcomposer.org/installer | php
+mkdir -p "$HOME/bin"
+mv composer.phar "$HOME/bin/composer"
+PATH="$HOME/bin:$PATH"
+
+# go to the repo root
+cd $(dirname $0)/../../..
+
+cd php
+
+composer install
+composer test
+composer test_c

kokoro/linux/aarch64/test_php_aarch64.sh

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+#!/bin/bash
+
+set -ex
+
+# go to the repo root
+cd $(dirname $0)/../../..
+
+# there is no php testing docker image readily available, so we build
+# our own. It's a aarch64 image, but that's fine since qemu will
+# automatically be used to run the commands in the dockerfile.
+docker build -t testimage_protobuf_php_arm64v8 kokoro/linux/aarch64/testimage_protobuf_php_arm64v8
+
+if [[ -t 0 ]]; then
+  DOCKER_TTY_ARGS="-it"
+else
+  # The input device on kokoro is not a TTY, so -it does not work.
+  DOCKER_TTY_ARGS=
+fi
+
+# crosscompile protoc as we will later need it for the php build.
+# we build it under the dockcross/manylinux2014-aarch64 image so that the resulting protoc binary is compatible
+# with a wide range of linux distros (including any docker images we will use later to build and test php)
+kokoro/linux/aarch64/dockcross_helpers/run_dockcross_manylinux2014_aarch64.sh kokoro/linux/aarch64/protoc_crosscompile_aarch64.sh
+
+# use an actual aarch64 docker image (with a real aarch64 php) to run build & test protobuf php under an emulator
+# * mount the protobuf root as /work to be able to access the crosscompiled files
+# * to avoid running the process inside docker as root (which can pollute the workspace with files owned by root), we force
+#   running under current user's UID and GID. To be able to do that, we need to provide a home directory for the user
+#   otherwise the UID would be homeless under the docker container and pip install wouldn't work. For simplicity,
+#   we just run map the user's home to a throwaway temporary directory
+docker run $DOCKER_TTY_ARGS --rm --user "$(id -u):$(id -g)" -e "HOME=/home/fake-user" -v "$(mktemp -d):/home/fake-user" -v "$(pwd)":/work -w /work testimage_protobuf_php_arm64v8 kokoro/linux/aarch64/php_build_and_run_tests_with_qemu_aarch64.sh

kokoro/linux/aarch64/testimage_protobuf_php_arm64v8/Dockerfile

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+FROM arm64v8/debian:buster
+
+RUN apt-get update && apt-get install -y php7.3-cli php7.3-dev php7.3-bcmath composer phpunit curl git valgrind && apt-get clean