raid6: Add RISC-V SIMD syndrome and recovery calculations

The assembly is originally based on the ARM NEON and int.uc, but uses
RISC-V vector instructions to implement the RAID6 syndrome and
recovery calculations.

The functions are tested on QEMU running with the option "-icount shift=0":

  raid6: rvvx1    gen()  1008 MB/s
  raid6: rvvx2    gen()  1395 MB/s
  raid6: rvvx4    gen()  1584 MB/s
  raid6: rvvx8    gen()  1694 MB/s
  raid6: int64x8  gen()   113 MB/s
  raid6: int64x4  gen()   116 MB/s
  raid6: int64x2  gen()   272 MB/s
  raid6: int64x1  gen()   229 MB/s
  raid6: using algorithm rvvx8 gen() 1694 MB/s
  raid6: .... xor() 1000 MB/s, rmw enabled
  raid6: using rvv recovery algorithm

[Charlie: - Fixup vector options]

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20250305083707.74218-1-zhangchunyan@iscas.ac.cn


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>