linux: move DCCP_ constants from linux/gnu to linux

[nekopsykose]

closes #3132

these constants come from linux headers, so they should be exposed for "linux", not just glibc.

this change exposes them for linux/musl and linux/uclibc.

of note, android contains these same constants, but moving it to linux-like would also expose them on emscripten, which does not have it.

(not sure if things are placed in the correct place, but this looks like an okayish start)

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[nekopsykose]

not sure if the failing freebsd-14 ci is related to any of this

[zonyitoo]

@JohnTitor Could you rerun the CI? I don't think there is anything related to freebsd in this PR.

[JohnTitor]

Re-running doesn't help it here as the failure is valid, it has been fixed on master already so we just need
@bors r+

[bors]

📌 Commit b5c207a has been approved by JohnTitor

It is now in the queue for this repository.

[bors]

⌛ Testing commit b5c207a with merge 71ec1cf...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13, checks-cirrus-freebsd-14
Approved by: JohnTitor
Pushing 71ec1cf to master...