[arm/gcc] Don't rely on KUSER_HELPERS feature for atomics

This affects cases with leading capital letters.

This breaks compatibility with previous C# releases, but
fixes compatibility with other implementations.

See #2278 for details.

generic atomicops: Use strong compare_exchange

Fix compiler warnings when running :protobuf_test

When running >bazel test :protobuf_test a few compiler warnings came up.  They were centered around the usage of "int" as the loop counter where the value should have been a size_t.

Docs: Link to Apple's Swift plugin and runtime for protobufs

Add note about extension use and Coding support.

Weak compare-and-exchange are allowed to fail spuriously, so we have
to use the strong variation here.

ARM specific CONFIG_KUSER_HELPERS feature can be disabled in Linux kernel,
and in this case, we shouldn't crash. Use gcc built-in functions instead
of arm specific code if they are available.

Adds pushLimit: and popLimit: into GPBCodedInputStream

define no_threadlocal on OpenBSD

Support extra parameters for plugins.

Hacking ByteBufferWriter to work with GAE

Fixes #2269

Place Python extensions correctly in Bazel build.

Use git clean before installing via bundler

Need to expose generated protobuf C++ headers

Merge 3.1.x branch into master.

Declare all inputs of protoc action

ctx.executable.plugin must be in the inputs of protoc's action when using a plugin, otherwise the action will fail.

This bug has been hidden by a bug in Bazel: for every ctx.action, Bazel used to automatically add the runfiles of all executable inputs of the RULE instead of using the inputs of the specific ACTION. Consequently, we could get away with underspecifying the inputs of the action.

Fix MSVC build when HAVE_LONG_LONG is defined.

Merge 3.1.x branch into master.

Fix python_cpp test on Mac. Link staticly when building extension, so that the extension doesn't require installing protobuf library. (#2232)