Added support for building the aarch64 and ppcle64 protoc binaries with Kokoro

This was fairly straightforward using the existing build-protoc.sh
script. The only problem I ran into was that the x86 Docker builds
create output directories owned by root, which caused some permission
issues. Fortunately it was easy to get around that just by doing those
Docker builds last.

Merge 3.6.x branch into master

tests: fix link failure and stack overflow on Mingw w64

* Ignore unknown enum received in json when ignoreUnknownFields flag is set.

fix third_party/benchmark submodule init 

I have added the xsrpcj RPC implementation for java

I made a few small fixes to the documentation related to publishing
protoc artifacts:
- The target directory for Mac should be called osx instead of macos.
- There needs to be a directory for aarch_64.
- We need to avoid calling "mvn clean" inside the protoc-artifacts
  directory, since that will delete the contents of the target/
  subdirectory.

* Give a unique category to each test.

This change introduce a TestCategory enum to ConformanceRequest. Existing tests
are divided into three categories: binary format test, json format test and json
format (ignore unknown when parsing) test. For the previous two categories, there
is no change to existing testee programs. For tests with the last category, testee programs
should either enable ignoring unknown field during json parsing or skip the test.

* Fix python test

* Fix java

* Fix csharp

* Update document

* Update csharp generated code

* Add conformance test for php c back

php c extension has different result for conformance test for different
php version and architecture. Try to add conformance back for php 7.1 c extension first.

* Disable conformance test for c extension on 32-bit architecture
32-bit and 64-bit have different failing tests

* Fix typo

* Store the class object itself

* Add test for backwards compatibility

* Rename and add test

* Add comments

Added support for Visual Studio 2017 in gitignore

* Prevents installation of `phpunit/phpunit:^6` because the test cases in this project require `\PHPUnit_Framework_TestCase` which was removed in v6. Moves generated PHP classes for tests into autoload-dev so that they are not referenced in the dist autoload. Adds Composer scripts to run protoc with and without grpc.

* update composer test target to use the same commands as generate_php_test_proto in tests.sh

* Check the message to be encoded is the wrong type. (#4885)

* Change TypeError to ArgumentError

* Remove/replace travis references.

Added Netifi Proteus to Registered Extensions

* php: Added nanosecond support for Timestamp

* php: Fixed compatibility test

This change only makes assertEquals pass for message in c extension. However, it actually does nothing.
This is the same behavior before 3.6.0 release.

When running unit tests, read reference files in text mode

This should put an end to all of the reports of unit test failures on
Windows machines.

* Delete unused files.

Sync internal benchmark changes

* upgrade dotnet SDK to 2.1.3 on kokoro linux

Cherry-pick master build fixes and kokoro changes to 3.6.x

* Add continuous test for ruby 2.3, 2.4 and 2.5

* Change ruby 2.5 to 2.5.0

* No need to provide argument to rb_funcall when argc is 0

* Fix tests for ruby 2.5

* Use rescue instead of assert_raise to accept subclass of error

This avoids the need to use "yum update && yum upgrade" in the container
to be able to contact GitHub, which requires TLS 1.2[1].

I have verified that binaries built with this container still run in the
previous container; no errors like "/lib64/libc.so.6: version
`GLIBC_2.14' not found", which occur if using too new of a glibc when
compiling. CentOS 6.6 has glibc version 2.12 release 1.209.el6. CentOS
6.9 has glibc version 2.12 release 1.149.el6. Both would upgrade to
release 1.212.el6 via "yum update && yum upgrade".

1. https://githubengineering.com/crypto-deprecation-notice/

Now that we depend on C++11, we need at least GCC 4.8 instead of 4.7.
This change updates the Docker setup to continue using CentOS 6.6 but
with GCC 4.8.

I also added libm to the whitelist for dynamically linked libraries for
the ARM64 protoc binary.

* Remove js_embed binary.