WIP: Implement I/O event manager based on io_uring

This MR adds an I/O event manager backend based on the io_uring interface available in modern Linux kernels. The relevant code is in /libraries/base/GHC/Event/IoUring.hsc. It also contains the following changes to support the new backend:

• Port over the io_uring bindings from @bgamari into base.
• Add a new HAVE_IO_URING check similar to HAVE_EPOLL, HAVE_KQUEUE etc. All changes are no-opped if this flag is not set.

Linked issue: #18390. NOT included yet is a unit test, since I was not sure what to test and how the CI environment looks (is io_uring even available there?). It does work however, both "on my machine" and on AWS (blog post about test results).

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• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
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• updates the users guide if applicable
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