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Follow up to LR_02_050: Instrinsify math/bits on riscv64 which intrinsified some but not all of math.bits. This issues covers the work needed to accelerate Bswap and OnesCount on riscv64.

Dependency
Upstreaming	COMPLETED	cmd/compile: intrinsify math/bits.Bswap on riscv64 cmd/compile,internal/cpu,runtime: intrinsify math/bits.OnesCount on riscv64
Upstream version
Contacts	Mark Ryan

LR_02_051: Instrinsify math/bits on riscv64 (Part 2)