Cherry-pick Objc changes

- The Timestamp proto does not allow for negative nanos fields, so the seconds
  must be shifted and a positive nanos then applied.
- Tweak the helpers on Duration to make it clear there is no "base" time
  involved.
- Update the unittests for duration and timestamp to cover positive and
  negative NSTimeIntervals and what their impact is on the protos.

The Protobuf library doesn’t require the 7.1 deployment target so
reverting it back to 7.0

* Update upb, fixes some bugs (including a hash table problem).

* Ruby: added a test for the previous hash table corruption.

Verified that this triggers the bug in the currently released
version.

* Ruby: bugfix for SEGV.

* Ruby: removed old code for dup'ing defs.

To make the test pass I needed to fix out-of-tree builds and update
EXTRA_DIST and CLEANFILES.

* Fix generation of extending nested messages in JavaScript

* Added missing test8.proto to build

- If setting/clearing a repeated field/map that was objects, check the class
  before checking the autocreator.
- Just to be paranoid, don’t mutate within copy/mutableCopy for the autocreated
  classes to ensure there is less chance of issues if someone does something
  really crazy threading wise.
- Some more tests for the internal AutocreatedArray/AutocreatedDictionary
  classes to ensure things are working as expected.
- Add Xcode 8.2 to the full_mac_build.sh supported list.

Pull request #2517 caused cross compilations to start failing, because
the js_embed binary was being built to run on the target platform
instead of on the build machine. This change updates the Autotools build
to use the AX_PROG_CXX_FOR_BUILD macro to find a suitable compiler for
the build machine and always use that when building js_embed.

Restore include style fix (e3da722b) that has been trampled by
auto-generation of well_known_types_embed.cc

embed.cc: In function ‘std::string CEscape(const string&)’:
embed.cc:51:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < str.size(); ++i) {
                                ^

* fix int64 decoding

* fix int64 decoding + tests

Bump version number to 3.2.0

In pull request #2517 I made this change for the CMake and autotools
builds but forgot to do it for the Bazel build.

Symbol is not yet available on older versions of Node.js and so this
test fails with them. This change just directly checks whether Symbol is
available before we try to use it.

on Mac.

Auto-generate well_known_types_embed.cc

The check target did not depend on the test_plugin binary, so this would
cause "make check" to fail at times because of the absence of
test_plugin. This change adds a dependency on test_plugin so that it
will always get built before the tests are executed.