add xuantie toolchain #147

Job	Run time
build-linux-apps (cpp)	1m 15s
build-smart-apps (aarch64)	7m 19s
build-smart-apps (arm)	7m 27s
build-linux-apps (hello)	1m 6s
build-smart-apps (riscv64gc)	6m 43s
build-linux-apps (micropython)	1m 27s
build-linux-apps (zlib)	1m 0s
build-smart-apps (xuantie)	2m 1s
build-linux-apps (busybox)	1m 46s
	30m 4s

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