Fix the definition of domainset_t in 32-bit FreeBSD

It's always had the wrong size, but apparently never been tested on
32-bit FreeBSD.

In addition to fixing its size, it ought to be moved info freebsd/mod.rs
.  Otherwise it's pretty much inaccessible to everyone.

https://github.com/freebsd/freebsd-src/blob/main/sys/sys/_domainset.h

(backport <rust-lang#3948>)
(cherry picked from commit 19d213d)

src/unix/bsd/freebsdlike/freebsd/freebsd13/mod.rs

@@ -41,7 +41,10 @@ s! {
     }
 
     pub struct __c_anonymous_domainset {
-        _priv: [::uintptr_t; 4],
+        #[cfg(target_pointer_width = "64")]
+        _priv: [::c_ulong; 4],
+        #[cfg(target_pointer_width = "32")]
+        _priv: [::c_ulong; 8],
     }
 
     pub struct kinfo_proc {

src/unix/bsd/freebsdlike/freebsd/freebsd14/mod.rs

@@ -41,7 +41,10 @@ s! {
     }
 
     pub struct __c_anonymous_domainset {
-        _priv: [::uintptr_t; 4],
+        #[cfg(target_pointer_width = "64")]
+        _priv: [::c_ulong; 4],
+        #[cfg(target_pointer_width = "32")]
+        _priv: [::c_ulong; 8],
     }
 
     pub struct kinfo_proc {

src/unix/bsd/freebsdlike/freebsd/freebsd15/mod.rs

@@ -41,7 +41,10 @@ s! {
     }
 
     pub struct __c_anonymous_domainset {
-        _priv: [::uintptr_t; 4],
+        #[cfg(target_pointer_width = "64")]
+        _priv: [::c_ulong; 4],
+        #[cfg(target_pointer_width = "32")]
+        _priv: [::c_ulong; 8],
     }
 
     pub struct kinfo_proc {