[libc] Revisit syscall tests that have different behaviors under QEMU.

The following tests have different behaviors under qemu-riscv64:
```
sys/mman/linux/madvise_test
sys/mman/linux/mincore_test
sys/mman/linux/mlock_test
sys/mman/linux/mremap_test
sys/mman/linux/posix_madvise_test
socket/linux/sendrecvmmsg_test
socket/linux/socketopt_test
socket/linux/socketpair_test
unistd/pread_pwrite_test
unistd/read_write_test
```
They are accommodated in #208920 .

Review these tests again after June 2028 to see if those exceptions are still needed.

After June 2028, start with the listed sys/mman/linux, socket/linux, and unistd tests and review their behavior under qemu-riscv64. Compare the existing exceptions accommodated in #208920 with current results; done means determining whether each exception is still needed and updating the tests accordingly.

Written by the indexing model from the issue text.