libc.src/sys/socket/linux/libc.test.src.sys.socket.linux.sockatmark_test failing on `release/23.x` branch

While trying to test https://github.com/llvm/llvm-project/pull/217720 I hit a test failure that appears unrelated.

https://github.com/llvm/llvm-project/actions/runs/32403041612/job/96535529431?pr=217720

`libc.src/sys/socket/linux/libc.test.src.sys.socket.linux.sockatmark_test`
```
[32m[==========] [0mRunning 3 tests from 1 test suite.
[32m[ RUN ] [0mLlvmLibcSockatmarkTest.SocketpairReturnsFalse
/home/gha/actions-runner/_work/llvm-project/llvm-project/libc/test/src/sys/socket/linux/sockatmark_test.cpp:37: FAILURE
Failed to match LIBC_NAMESPACE::sockatmark(sockpair[0]) against Succeeds(0).
Expected return value to be equal to 0 but got -1.
Expected errno to be equal to 0(0) but got ENOTSUP(95).
[31m[ FAILED ] [0mLlvmLibcSockatmarkTest.SocketpairReturnsFalse
[32m[ RUN ] [0mLlvmLibcSockatmarkTest.InvalidFdFails
[32m[ OK ] [0mLlvmLibcSockatmarkTest.InvalidFdFails (0 ns)
[32m[ RUN ] [0mLlvmLibcSockatmarkTest.NonSocketFdFails
[32m[ OK ] [0mLlvmLibcSockatmarkTest.NonSocketFdFails (7 us)
Ran 3 tests. PASS: 2 FAIL: 1
```

Start with libc/test/src/sys/socket/linux/sockatmark_test.cpp and reproduce libc.src/sys/socket/linux/libc.test.src.sys.socket.linux.sockatmark_test on the release/23.x branch. Investigate why sockatmark on the socket pair returns ENOTSUP, then rerun the three-test suite and confirm the socket-pair case passes without regressing the invalid-fd and non-socket cases.

Written by the indexing model from the issue text.