Incompatibility (build failure) with Linux distros using kernel versions < 5.11

Since `swift-system` 1.7.0, more specifically #329, the `io_uring` implementation has used the `IORING_ENTER_EXT_ARG` flag, which is only supported in Linux kernel version 5.11 or newer. This breaks build compatibility with distros running older kernels, such as Oracle Linux 8 (kernel 5.4.17), as the `linux/io_uring.h` header does not include the flag. Any project which uses `swift-system`, even indirectly (such as via NIO), is affected.

Start by examining the io_uring implementation and the use of IORING_ENTER_EXT_ARG, then compare it with the linux/io_uring.h header available on kernels older than 5.11. Reproduce the build on a system such as Oracle Linux 8 with kernel 5.4.17. Done means swift-system and projects that use it can build with older Linux kernel headers without losing supported io_uring behavior.

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