arm64: insn: Add encoders for LDRSB/LDRSH/LDRSW

To support BPF sign-extend load instructions, add encoders for
LDRSB/LDRSH/LDRSW.

LDRSB/LDRSH/LDRSW (immediate) is encoded as follows:

     3     2 2   2   2                       1         0         0
     0     7 6   4   2                       0         5         0
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | sz|1 1 1|0|0 1|opc|        imm12          |    Rn   |    Rt   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LDRSB/LDRSH/LDRSW (register) is encoded as follows:

     3     2 2   2   2 2         1     1 1   1         0         0
     0     7 6   4   2 1         6     3 2   0         5         0
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | sz|1 1 1|0|0 0|opc|1|    Rm   | opt |S|1 0|    Rn   |    Rt   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

   - sz
     indicates whether 8-bit, 16-bit or 32-bit data is to be loaded

   - opc
     opc[1] (bit 23) is always 1 and opc[0] == 1 indicates regsize
     is 32-bit. Since BPF signed load instructions always exend the
     sign bit to bit 63 regardless of whether it loads an 8-bit,
     16-bit or 32-bit data. So only 64-bit register size is required.
     That is, it's sufficient to set field opc fixed to 0x2.

   - opt
     Indicates whether to sign extend the offset register Rm and the
     effective bits of Rm. We set opt to 0x7 (SXTX) since we'll use
     Rm as a sgined 64-bit value in BPF.

   - S
     Optional only when opt field is 0x3 (LSL)

In short, the above fields are encoded to the values listed below.

                   sz   opc  opt   S
LDRSB (immediate)  0x0  0x2  na    na
LDRSH (immediate)  0x1  0x2  na    na
LDRSW (immediate)  0x2  0x2  na    na
LDRSB (register)   0x0  0x2  0x7   0
LDRSH (register)   0x1  0x2  0x7   0
LDRSW (register)   0x2  0x2  0x7   0

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Florent Revest <revest@chromium.org>
Acked-by: Florent Revest <revest@chromium.org>
Link: https://lore.kernel.org/bpf/20230815154158.717901-2-xukuohai@huaweicloud.com