Set `target_vendor = "openwrt"` on `mips64-openwrt-linux-musl` OpenWRT is a Linux distribution for embedded network devices. The target name contains `openwrt`, so we should set `cfg(target_vendor = "openwrt")`. This is similar to what other Linux distributions do (the only one in-tree is `x86_64-unikraft-linux-musl`, but that sets `target_vendor = "unikraft"`). Motivation: To make correctly [parsing target names](https://github.com/rust-lang/cc-rs/pull/1413) simpler. Fixes https://github.com/rust-lang/rust/issues/131165. CC target maintainer `@Itus-Shield`