add support for loongarch64 #543

Job	Run time
prepare (mainline)	6m 54s
prepare (buildkit)	7m 5s
prepare (desktop)	7m 23s
e2e (mainline, pip-youtube-dl, linux/arm/v7)	1m 19s
test (mainline)	2m 56s
e2e (mainline, webpack, linux/arm/v7)	1m 49s
validate (./hack/lint)	24s
test (buildkit)	25s
e2e (desktop, pip-youtube-dl, linux/arm/v7)	1m 9s
validate (./hack/validate-vendor)	16s
test (desktop)	12s
e2e (desktop, webpack, linux/arm/v7)	1m 51s
validate (./hack/install-and-test)	47s
e2e (mainline, ejabberd, linux/arm64)	13m 33s
e2e (desktop, tini, linux/arm64, true)	34s
build (mainline)	27m 11s
build (buildkit)	26m 49s
build (desktop)	27m 10s
	2h 7m 47s

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