libuv: Explicitly enable GNU APIs on Cygwin

Backport upstream libuv PR 2085 to define _GNU_SOURCE.

changed milestone to %4.0.0

added os:cygwin triage:env-regression workflow:in-review labels

assigned to @brad.king

Do: stage

added workflow:nightly-testing label and removed workflow:in-review label

Do: merge

mentioned in commit 6cfbb20c

merged manually

removed workflow:nightly-testing label