sysconfig.get_platform(): specify result for Windows ARM64

[ncoghlan]

https://docs.python.org/3/library/sysconfig.html#sysconfig.get_platform currently says for Windows:

Windows will return one of:

• win-amd64 (64bit Windows on AMD64, aka x86_64, Intel64, and EM64T)
• win32 (all others - specifically, sys.platform is returned)

Since #77306 was implemented, ARM64 is a possibility on Windows as well, so the expected platform identifier should be listed in the docs.

Linked PRs

• gh-128696: Add arm64 to the get_platform return val description #128701

[rruuaanng]

If you don't mind, I'll add it today :)