精简指令集合中读/写记忆体指令 Lhu--Load Half Word Unsigned
指令格式：Lhu Rt,I(Rs) Rt <- M(Rs+I)
sign extended immidiate. Two bytes from storage is
loaded into the 2 less significant bytes of Rt, other
bytes are set to zero.
在risc-v指令中，
lhu 为I类型,OPCODE: 0000011, FUNC3 101。
使用格式为：R[rd]={48'b0,M[R[rs1]+imm](15:0)}
