libc-test: add mips to the sys_memfd_secret skip list

[chenx97]

Description

Existing MIPS ABIs don't have memfd_secret. Let's skip it for tests.

Sources

mips linux syscall tables https://github.com/torvalds/linux/tree/0d97f2067c166eb495771fede9f7b73999c67f66/arch/mips/kernel/syscalls

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[JohnTitor]

Looks good, thank you!