[PATCH] cipher:riscv: gate Zvkned AES backend on VLEN == 128

[Michael Neuling]

Jussi,

To try to eliminate qemu and gcc, I've done some more testing:

I run this test case on a Banana BPI-F3 with Spacemit X60 cores (RVA22 +
RVV 1.0 with VLEN=256) and it also fails there. This was compiled with gcc 14.2.

The earlier qemu test I did with gcc 15.0 and gcc 13.3. Both fail with VLEN=256.

SpacemiT X60 result:

% ./libgcrypt-rvv-vlen128-assumption
Element-by-element view of out[0..15]:

  out[ 0] = 10001111
  out[ 1] = 10002222
  out[ 2] = 10003333
  out[ 3] = 10004444
  out[ 4] = 00000000
  out[ 5] = 00000000
  out[ 6] = 00000000
  out[ 7] = 00000000
  out[ 8] = 20001111
  out[ 9] = 20002222
  out[10] = 20003333
  out[11] = 20004444
  out[12] = 00000000
  out[13] = 00000000
  out[14] = 00000000
  out[15] = 00000000

libgcrypt-shaped layout (VLEN=128 assumption): BUG -- AES_CRYPT m4
vl=16 will not find the 4 blocks here

Where each loaded m1 register actually lands in g (per RVV intrinsic
spec, sub-register N -> elements N*VLMAX_m1 .. (N+1)*VLMAX_m1 - 1):
  out[0..3]   = sub-register 0 (= r0 + r0-tail)
  ... and so on for sub-registers 1..3
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