ORC-2216: [C++] Add RVV bit-unpacking for RLEv2

# Add  RVV acceleration for RLEv2 bit unpacking

The scalar RLEv2 decoder is a  bottleneck when we're reading wide integer columns. Since RVV hardware is becoming much more common, this patch adds a vectorized path to unpack bits in parallel.

## Changes
- Add `c++/src/BpackingRvv.{hh,cc}`: Covers bit widths 3..32, 40, 48 and 56 using gather-based and strided-load vector paths; falls back to the scalar implementation for widths 1, 2, 64 and for bit-level misalignment / buffer boundaries.
- `c++/src/Dispatch.hh`: add `DispatchLevel::RVV` and the corresponding `isSupported` check.
- `c++/src/RleDecoderV2.cc`: register the RVV implementation in the dispatch table when `ORC_HAVE_RUNTIME_RVV` is defined.
- `c++/src/CMakeLists.txt` and `c++/test/CMakeLists.txt`: compile the new source under RVV builds and reuse `TestRleVectorDecoder.cc` for vector decoder test coverage.

## Test

1. Build with `-DBUILD_ENABLE_RVV=ON` on a RISC-V machine(SG2044) with Gcc support RVV(I used gcc 14.2.1).
2. use ORC_USER_SIMD_LEVEL=RVV ./orc-test and ORC_USER_SIMD_LEVEL=NONE ./orc-test，all unittest pass.
3. I write a small standalone benchmark(bench_rvv.cc) to compare scalar and RVV decoding speeds. Switch paths via `ORC_USER_SIMD_LEVEL=NONE|RVV`. but i dont left `bench_rvv.cc` at the repo.

 ## performance
| bits | NONE_Mvals/s | RVV_Mvals/s | speedup |
|------|-------------|-------------|---------|
| 1    | 80.8        | 78.5        | 0.97    |
| 2    | 106.2       | 102.4       | 0.96    |
| 3    | 114.1       | 160.8       | 1.41    |
| 4    | 217.9       | 235.4       | 1.08    |
| 5    | 96.2        | 240.1       | 2.50    |
| 6    | 79.9        | 245.3       | 3.07    |
| 7    | 75.4        | 246.7       | 3.27    |
| 8    | 237.3       | 276.0       | 1.16    |
| 9    | 70.8        | 175.0       | 2.47    |
| 10   | 69.5        | 124.3       | 1.79    |
| 12   | 60.7        | 110.7       | 1.82    |
| 14   | 52.8        | 102.1       | 1.93    |
| 16   | 184.6       | 238.1       | 1.29    |
| 17   | 55.4        | 173.9       | 3.14    |
| 20   | 48.2        | 173.9       | 3.61    |
| 24   | 156.3       | 176.9       | 1.13    |
| 26   | 41.3        | 135.0       | 3.27    |
| 28   | 35.2        | 135.6       | 3.85    |
| 30   | 32.3        | 135.9       | 4.21    |
| 32   | 155.2       | 162.0       | 1.04    |
| 40   | 139.7       | 146.6       | 1.05    |
| 48   | 126.8       | 134.7       | 1.06    |
| 56   | 108.3       | 100.1       | 0.92    |
| 64   | 111.0       | 111.3       | 1.00    |
## Screenshot
## unittest
<img width="1340" height="495" alt="功能测试" src="https://github.com/user-attachments/assets/384ec71d-9150-4885-9657-7a5402c03096" />

## performance
<img width="708" height="1260" alt="性能测试" src="https://github.com/user-attachments/assets/d4c9956d-357d-4d14-82ac-3313120568fe" />

## bench_rvv.cc
<img width="1136" height="1861" alt="bench_rvv cc" src="https://github.com/user-attachments/assets/0affb9c6-6762-474b-9222-5a31b5b30ef6" />

Closes #2706 from zhanchangbao-sanechips/rvvopt.

Authored-by: changbao zhan <zhan.changbao@sanechips.com.cn>
Signed-off-by: Gang Wu <ustcwg@gmail.com>

Author: zhanchangbao-sanechips