feat(runtime): Add an OpenBSD implementation for rss() (#19221)

In case you would consider including specific implementations and not
only build fixes, here's the implementation of the ```rss()``` function
for OpenBSD.

runtime/ops/os/mod.rs

@@ -380,6 +380,53 @@ fn rss() -> usize {
   task_info.resident_size as usize
 }
 
+#[cfg(target_os = "openbsd")]
+fn rss() -> usize {
+  // Uses OpenBSD's KERN_PROC_PID sysctl(2)
+  // to retrieve information about the current
+  // process, part of which is the RSS (p_vm_rssize)
+
+  // SAFETY: libc call (get PID of own process)
+  let pid = unsafe { libc::getpid() };
+  // SAFETY: libc call (get system page size)
+  let pagesize = unsafe { libc::sysconf(libc::_SC_PAGESIZE) } as usize;
+  // KERN_PROC_PID returns a struct libc::kinfo_proc
+  let mut kinfoproc = std::mem::MaybeUninit::<libc::kinfo_proc>::uninit();
+  let mut size = std::mem::size_of_val(&kinfoproc) as libc::size_t;
+  let mut mib = [
+    libc::CTL_KERN,
+    libc::KERN_PROC,
+    libc::KERN_PROC_PID,
+    pid,
+    // mib is an array of integers, size is of type size_t
+    // conversion is safe, because the size of a libc::kinfo_proc
+    // structure will not exceed i32::MAX
+    size.try_into().unwrap(),
+    1,
+  ];
+  // SAFETY: libc call, mib has been statically initialized,
+  // kinfoproc is a valid pointer to a libc::kinfo_proc struct
+  let res = unsafe {
+    libc::sysctl(
+      mib.as_mut_ptr(),
+      mib.len() as _,
+      kinfoproc.as_mut_ptr() as *mut libc::c_void,
+      &mut size,
+      std::ptr::null_mut(),
+      0,
+    )
+  };
+
+  if res == 0 {
+    // SAFETY: sysctl returns 0 on success and kinfoproc is initialized
+    // p_vm_rssize contains size in pages -> multiply with pagesize to
+    // get size in bytes.
+    pagesize * unsafe { (*kinfoproc.as_mut_ptr()).p_vm_rssize as usize }
+  } else {
+    0
+  }
+}
+
 #[cfg(windows)]
 fn rss() -> usize {
   use winapi::shared::minwindef::DWORD;