Support loongarch64

[k0tran]

Summary of the PR

This will help building codex (and other crates that depend on seccompiler) on loongarch64.
There is one problem with test_tsync which should be addressed by you.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

Testing

I tested this on loongarch64 machine with the following

cargo build
cargo build --release
cargo test -- --skip=test_tsync
cargo test --release -- --skip=test_tsync

it works

test_tsync

The error is:

thread 'test_tsync' panicked at tests/multi_thread.rs:37:5:
assertion `left == right` failed: there should be no errors
  left: 2
 right: 0

Which is reproducable with this:

fn main() {
    let errno2 = std::io::Error::last_os_error().raw_os_error().unwrap();
    println!("before: {errno2}"); // outputs 2
    let pid = unsafe { libc::getpid() };
    let errno = std::io::Error::last_os_error().raw_os_error().unwrap();
    println!("after: {errno}"); // outputs 2 again
}

What should be the correct way to handle it? Should getpid os error codes be checked at all? Man page says it's always succesfull.