runtime: add free_memory/0 implementation for FreeBSD too (fix #23581) (#23594)

vlib/runtime/free_memory_impl_freebsd.c.v

@@ -0,0 +1,22 @@
+module runtime
+
+fn C.sysctlnametomib(name charptr, mib &int, len &usize) int
+
+fn free_memory_impl() usize {
+	$if cross ? {
+		return 1
+	}
+	$if !cross ? {
+		$if freebsd {
+			page_size := usize(C.sysconf(C._SC_PAGESIZE))
+			mut mib := [4]int{}
+			mut len := usize(4)
+			unsafe { C.sysctlnametomib(c'vm.stats.vm.v_free_count', &mib[0], &len) }
+			mut free_pages := int(0)
+			bufsize := usize(4)
+			unsafe { C.sysctl(&mib[0], mib.len, &free_pages, &bufsize, 0, 0) }
+			return page_size * usize(free_pages)
+		}
+	}
+	return 1
+}