Implement RISC-V Zb intrinsics #1809

Job	Run time
Check Style	22s
Automatic intrinsic verification	41s
Env Override	17s
Build std_detect	1m 54s
Build Documentation	1m 35s
Test (i686-unknown-linux-gnu)	2m 59s
Test (x86_64-unknown-linux-gnu)	6m 45s
Test (x86_64-unknown-linux-gnu-emulated)	11m 40s
Test (arm-unknown-linux-gnueabihf)	2m 55s
Test (armv7-unknown-linux-gnueabihf)	16m 43s
Test (aarch64-unknown-linux-gnu)	43m 29s
Test (riscv64gc-unknown-linux-gnu)	2m 41s
Test (powerpc64le-unknown-linux-gnu)	6m 42s
Test (s390x-unknown-linux-gnu)	2m 57s
Test (wasm32-wasi)	3m 13s
Test (i586-unknown-linux-gnu)	3m 37s
Test (nvptx64-nvidia-cuda)	35s
Test (thumbv6m-none-eabi)	17s
Test (thumbv7m-none-eabi)	19s
Test (thumbv7em-none-eabi)	21s
Test (thumbv7em-none-eabihf)	24s
Test (aarch64-apple-darwin)	2m 33s
Test (x86_64-pc-windows-msvc)	12m 16s
Test (i686-pc-windows-msvc)	6m 28s
Test (x86_64-apple-darwin, macos-11)	9m 41s
	2h 21m 24s

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