This fixes the cosmopolitan platform.
- Switch `build_cosmocc.sh` and platform documentation to
explicitly use the x86_64 cosmocc compiler as multi-arch
cosmocc won't work here. Older version `cosmocc` just did
a x86_64 build.
- Add missing items from `platform_internal.h` to fix build.
This folder contains the platform abstract layer for multiple platforms. To support a new platform, you can simply create a new folder here and implement all the APIs defined in include folder.
Refer to port_wamr.md for how to port WAMR to a target platform.