runtime: add free_memory/0 implementation for OpenBSD (fix #23579) (#23583)

Author: lcheylus

vlib/runtime/free_memory_impl_openbsd.c.v

@@ -0,0 +1,15 @@
+module runtime
+
+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
+		}
+	}
+	return 1
+}