arm32, bpf: add support for sign-extension mov instruction

The cpuv4 added a new BPF_MOVSX instruction that sign extends the src
before moving it to the destination.

BPF_ALU | BPF_MOVSX sign extends 8-bit and 16-bit operands into 32-bit
operands, and zeroes the remaining upper 32 bits.

BPF_ALU64 | BPF_MOVSX sign extends 8-bit, 16-bit, and 32-bit  operands
into 64-bit operands.

The offset field of the instruction is used to tell the number of bit to
use for sign-extension. BPF_MOV and BPF_MOVSX have the same code but the
former sets offset to 0 and the later one sets the offset to 8, 16 or 32

The behaviour of this instruction is dst = (s8,s16,s32)src

On ARM32 the implementation uses LSH and ARSH to extend the 8/16 bits to
a 32-bit register and then it is sign extended to the upper 32-bit
register using ARSH. For 32-bit we just move it to the destination
register and use ARSH to extend it to the upper 32-bit register.

Signed-off-by: Puranjay Mohan <[email protected]>
Reviewed-by: Russell King (Oracle) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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