bpo-31904: support signal module for VxWorks

[pxinwr]

1. There are some discussion about "sig_atomic_t fd;" in PR 12670. POSIX spec doesn't define its type. In practice, most implementation defines it as int. Go to Atomic-Types So far it works fine on known OSes. However, on VxWorks, sig_atomic_t is defined as 'unsigned char'. Assigning it to -1 will have problem. So we define wakeup.fd as int specially for VxWorks.
2. VxWorks has no getitimer() and setitimer() C api available. So skipping the test on VxWorks.

https://bugs.python.org/issue31904

[vstinner]

I would prefer to not add a new blurb category just for VxWorks. Would you mind to reuse the Library category?

[pxinwr]

Fine. Fixed accordingly.

[vstinner]

I would prefer to not copy/paste the code, restrict the difference to the loop. Something like:

Suggested change

	for chunk_size in (2 ** 16, 2 ** 8, 1):
	if sys.platform == "vxworks":
	CHUNK_SIZES = (1,)
	else:
	CHUNK_SIZES = (2 ** 16, 2 ** 8, 1)
	for chunk_size in CHUNK_SIZES:

[pxinwr]

Better solution. Changed.

[vstinner]

Merged, thanks. The updated PR is simpler.