* Fix protoc release

* Update Dockerfile to include libatomic

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.

1. Changed maven script to only do artifact uploading and removed build
script invocation from it. We didn't use maven to invoke the build
script before (we built protoc manually and editted pom.xml to only do
uploading for previous releases), and will not use it in the future (we
will use kokoro to build artifacts).
2. Cleaned up build-protoc.sh and README.md: removed the part about
using maven to build and listed supported platforms explicitly.

1. Changed maven script to only do artifact uploading and removed build
script invocation from it. We didn't use maven to invoke the build
script before (we built protoc manually and editted pom.xml to only do
uploading for previous releases), and will not use it in the future (we
will use kokoro to build artifacts).
2. Cleaned up build-protoc.sh and README.md: removed the part about
using maven to build and listed supported platforms explicitly.

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.

Need to install mingw-w64 on linux to cross compile

"make google/protobuf/stubs/pbconfig.h" was added in hope of addressing
the issue that when you "make protoc" from a freshly checked out
project, pbconfig.h will be reported missing. However, the trick doesn't
seem to work. Instead, add instructions in the document to work the issue
around.

Also document why MSYS2 cannot be used for publishing protoc.

pbconfig.h has gone. This commit removes the target.

1. make google/protobuf/stubs/pbconfig.h before making protoc, otherwise it
won't build a freshly checked-out code.
2. Document the build environments that have been tested to work.
3. Add support for MINGW64

release.

- Do not close the staging repository automatically
- Added staging.repository property
- Updated README with instructions for deployment
- Fix building 32-bit Mac artifact

Run make protoc(.exe) instead of just make, because the latter fails on Cygwin with mingw gcc; build-protoc.sh works under Cygwin; build-protoc.sh will use the --host parameter consistent with what os-maven-plugin has detected