Auto merge of #137836 - madsmtm:openwrt-target-vendor, r=jieyouxu

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](rust-lang/cc-rs#1413) simpler.

Fixes rust-lang/rust#131165.

CC target maintainer `@Itus-Shield`

Author: bors