fix cross compiling build

Author: devnexen

Diff for: library/std/src/sys/pal/unix/thread.rs

@@ -854,11 +854,20 @@ pub mod guard {
             static mut GUARD_PAGES: usize = 1;
 
             ONCE.call_once(|| {
+                extern "C" {
+                    pub fn sysctlbyname(
+                        oid: *const libc::c_char,
+                        ov: *mut libc::c_void,
+                        osize: *mut libc::size_t,
+                        nv: *const libc::c_void,
+                        nsize: libc::size_t,
+                    ) -> libc::c_int;
+                }
                 let mut guard: usize = 0;
                 let mut size = crate::mem::size_of_val(&guard);
                 let oid = crate::ffi::CStr::from_bytes_with_nul(b"security.bsd.stack_guard_page\0")
                     .unwrap();
-                let res = libc::sysctlbyname(
+                let res = sysctlbyname(
                     oid.as_ptr(),
                     &mut guard as *mut _ as *mut _,
                     &mut size as *mut _ as *mut _,