bpo-31904: Add support for VxWorks RTOS

[pxinwr]

This PR enables cpython to cross-build on VxWorks RTOS. As part of the whole effort, this PR only aims at achieving some basic support including most of built-in modules, os and subprocess modules. More and full support will continuously added by the coming PRs.
VxWorks is a product developed and owned by Wind River. For VxWorks introduction or more details, go to https://www.windriver.com/products/vxworks/
Wind River will have a dedicated engineering team to contribute to the support as maintainers.
We already have a working buildbot worker internally, but has not bound to master. We will check the process for the buildbot, then add it.

https://bugs.python.org/issue31904

[the-knights-who-say-ni]

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[vstinner]

This PR is too big, it must be splitted into smaller parts:

• UTF-8 encoding
• subprocess + _vxapi module
• time changes: clock, mktime, etc.
• other misc changes: DELIM, configure, etc.

I'm interested to review the first one, UTF-8/encoding changes ;-)

[pxinwr]

This PR is too big, it must be splitted into smaller parts:

* UTF-8 encoding

* subprocess + _vxapi module

* time changes: clock, mktime, etc.

* other misc changes: DELIM, configure, etc.

I'm interested to review the first one, UTF-8/encoding changes ;-)

OK. I will close this big one and split it into several smaller ones. Thanks for your comments.