add support for loongarch64 #527

Job	Run time
prepare (mainline)	27s
prepare (buildkit)	13s
prepare (desktop)	24s
test (mainline)	3m 2s
test (buildkit)	25s
validate (./hack/lint)	29s
test (desktop)	11s
e2e (mainline, pip-youtube-dl, linux/arm/v7)	1m 12s
validate (./hack/validate-vendor)	18s
e2e (mainline, webpack, linux/arm/v7)	1m 53s
validate (./hack/install-and-test)	32s
e2e (desktop, pip-youtube-dl, linux/arm/v7)	1m 12s
e2e (desktop, webpack, linux/arm/v7)	1m 52s
e2e (mainline, ejabberd, linux/arm64)	13m 33s
e2e (desktop, tini, linux/arm64, true)	34s
build (mainline)	30m 30s
build (buildkit)	29m 24s
build (desktop)	31m 24s
	1h 57m 35s

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