add RVV support for MPI_OP

[heyujiao99]

This patch implements MPI_OP using RISC-V Vector (RVV) extensions.

Performance results:

• On older compilers (without RVV auto-vectorization):

  • RVV-optimized ops are 1.5-4x faster than C implementations.

• With GCC 14+ (RVV auto-vectorization enabled):

  • 2-buff RVV ops perform similarly to auto-vectorized C code.
  • 3-buff RVV ops are still 1.5-4x faster.