x/build: add LUCI linux-mips64le builder

[dmitshur]

There currently isn't a LUCI builder that tests the linux/mips64le port (other than the misc-compile builder, which tests only that the port compiles). This is the tracking issue for it.

The next steps that a builder owner will need to follow to make progress here are documented https://go.dev/wiki/DashboardBuilders#luci-builders.

CC @golang/mips.

[Rongronggg9]

Following the instructions at Dashboard builders:

hostname linux-mips64le-cip-united

CSR: linux-mips64le-cip-united.csr.txt

This builder is owned and hosted by CIP United Co. Ltd. (@CIP-United). We prefer the handle to be directly related to our company. If it is mandatory to use my username as the handle, please let us know, and we will regenerate the CSR.

[dmitshur]

That hostname should work well, thanks. Here is the corresponding certificate: linux-mips64le-cip-united-1761320497.cert.txt.

[Rongronggg9]

The builder is up. It is a systemd-nspawn container on a server equipped with Loongson 3A4000 (4 cores) and 8GiB RAM.