Add NXDK (Xbox) support

[ReenigneArcher]

This PR adds NXDK (Original Xbox) support. This is required for https://github.com/LizardByte/Moonlight-XboxOG

Requires cgutman/enet#24

[cgutman]

Is the NXDK version of MSVC too old for these intrinsics or something?

[ReenigneArcher]

The problem is that NXDK is using Clang targeting Win32. This defines _MSC_VER, but the intrinsics are not defined. Although the underlying functions do seem to be available.

https://github.com/XboxDev/nxdk/blob/fb5a9a7a58a431e8d70a9e7da87898059df376c0/lib/xboxrt/libc_extensions/stdlib_ext_.c#L73-L86

[ReenigneArcher]

I should add. I also had to make changes to NXDK to get everything working. I have not yet submitted these upstream, but will be doing so after changes here are approved.

XboxDev/nxdk@master...ReenigneArcher:nxdk:moonlight (This also includes changes required for compiling OpenSSL with NXDK)

[ReenigneArcher]

I should add. I also had to make changes to NXDK to get everything working. I have not yet submitted these upstream, but will be doing so after changes here are approved.

XboxDev/nxdk@master...ReenigneArcher:nxdk:moonlight (This also includes changes required for compiling OpenSSL with NXDK)

I've submitted required changes to nxdk via the following PRs:

• libc: Add POSIX headers & stubs XboxDev/nxdk#773 (required to compile moonlight-common-c and dependencies)
• lwip: Improve UDP buffering and thread priority XboxDev/nxdk#774 (required for streaming to actually work)