Disable pip cache when testing uploaded packages

Otherwise, pip complains "cannot find a version satisfies the
requirement." I suspect it was due to caching issues. I usued to
manually add a `sleep 30` between the upload and the installation test
which is quite unstable.

Update changelog for 3.5.1

Add support for Windows ARM64 build

Cherrypick for csharp, including:

Fix string::back() usage in googletest.cc

Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

Replace C++11 only method std::map::at

Update version number to 3.5.1

`umask 0022` makes sure the created package will be accessible by all users.

Avoid calling method from php extension directly

Create containing directory before generating well_known_types_embed.cc

Remove py2.6 support.

available.

Use matching enum type for IsPOD.

Explicitly propagate the status of Flush().

Before the change, an implicit Flush() will be triggered in the
destructor of the input stream. However, the return code of Flush() is
not discarded. This change makes sure when Flush() fails, we will
return false.

This fixes the following build error:

oldpwd=`pwd` && cd .../protobuf/src && \
$oldpwd/js_embed google/protobuf/compiler/js/well_known_types/any.js google/protobuf/compiler/js/well_known_types/struct.js google/protobuf/compiler/js/well_known_types/timestamp.js > $oldpwd/google/protobuf/compiler/js/well_known_types_embed.cc
/bin/bash: line 1: .../protobuf/target/src/google/protobuf/compiler/js/well_known_types_embed.cc: No such file or directory
Makefile:8201: recipe for target 'google/protobuf/compiler/js/well_known_types_embed.cc' failed

which is observed during the cross-compilation since the version 3.2.

This changes the linking error if php extension is not statically linked
to a runtime error. In this way, users who don't need Timestamp can
still use protobuf even if date extension is not statically linked in
php.

Add backslach to make class explict in global namespace

can be used at install time.

Merge 3.5.0.1 to 3.5.x

* Fix ruby gc_test in ruby 2.4

* Initialize global variables to Qnil.

* Add discard unknown API in ruby.

* Add test for oneof message field.

* Add TestUnknown to represent unknown field data clearly.

* Only serialize the message with unknown fields itself in test.

* Move discard_unknown from Message to Google.Protobuf