Add vec_xst #1903

Job	Run time
Check Style	11s
Build Documentation	1m 3s
Build std_detect	1m 2s
Env Override	13s
Automatic intrinsic verification	25s
Test (i686-unknown-linux-gnu)	1m 58s
Test (x86_64-unknown-linux-gnu)	3m 50s
Test (x86_64-unknown-linux-gnu-emulated)	9m 21s
Test (arm-unknown-linux-gnueabihf)	1m 15s
Test (armv7-unknown-linux-gnueabihf)	10m 54s
Test (aarch64-unknown-linux-gnu)	21m 49s
Test (riscv64gc-unknown-linux-gnu)	1m 23s
Test (powerpc64le-unknown-linux-gnu)	4m 18s
Test (s390x-unknown-linux-gnu)	1m 24s
Test (wasm32-wasi)	1m 31s
Test (i586-unknown-linux-gnu)	1m 42s
Test (nvptx64-nvidia-cuda)	25s
Test (thumbv6m-none-eabi)	14s
Test (thumbv7m-none-eabi)	15s
Test (thumbv7em-none-eabi)	14s
Test (thumbv7em-none-eabihf)	14s
Test (aarch64-apple-darwin)	2m 4s
Test (x86_64-pc-windows-msvc)	6m 27s
Test (i686-pc-windows-msvc)	2m 47s
Test (x86_64-apple-darwin, macos-11)	7m 34s
	1h 22m 33s

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