Preliminary native windows support, part 2: standard library and libgerbil

[rujialiu]

Much more changes this time :)

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[vyzo]

@fare first pass again?

[rujialiu]

General notes

1. We need win32ports's sys_time_h (https://github.com/win32ports/sys_time_h) and unistd_h (https://github.com/win32ports/unistd_h). That means I need to specify an external include path (in theory msvc can have "default include path" but most of the time we want to avoid it). Currently many codes are compiled even without using the default cc options form environment, so I had to add non-posix-extra-gsc-options to a few places. I didn't use cond-expand because I believe "respect default cc-options from environment variables" is also a correct dehavior for posix systems.
2. There are quite a bit of code duplications but I decided to keep them because it'll make making platform-specific changes easier (and with more confidence)