runtime: improve free_memory implementation for OpenBSD, by getting the stats from its UVM system (#24431)

Author: lcheylus

vlib/runtime/free_memory_impl_openbsd.c.v

@@ -1,14 +1,24 @@
 module runtime
 
+#include <sys/sysctl.h>
+#include <uvm/uvmexp.h>
+
+struct C.uvmexp {
+	pagesize int
+	free     int
+}
+
 fn free_memory_impl() usize {
 	$if cross ? {
 		return 1
 	}
 	$if !cross ? {
 		$if openbsd {
-			page_size := usize(C.sysconf(C._SC_PAGESIZE))
-			av_phys_pages := usize(C.sysconf(C._SC_AVPHYS_PAGES))
-			return page_size * av_phys_pages
+			mib := [C.CTL_VM, C.VM_UVMEXP]!
+			mut uvm := C.uvmexp{0, 0}
+			mut len := sizeof(C.uvmexp)
+			unsafe { C.sysctl(&mib[0], mib.len, &uvm, &len, C.NULL, 0) }
+			return usize(uvm.pagesize * uvm.free)
 		}
 	}
 	return 1