Short title (summary): Fix RISC-V compilation without RVV

Description

• Add RISC-V RVV support for find_next_json_quotable_character #2617 introduced an RVV check that only checks if the compiler supports RVV intrinsics without checking if RVV is enabled at compile time.
• This has caused compilation failure of node.js on riscv64 when RVV is not enabled. Fix it by appending a check for __riscv_vector.

Type of change

• Bug fix
• Optimization
• New feature
• Refactor / cleanup
• Documentation / tests
• Other (please describe):

How to verify / test

• Build node.js with simdjson 4.4.1 with -march=rv64gc in CFLAGS.

Please read before contributing:

• CONTRIBUTING: https://github.com/simdjson/simdjson/blob/master/CONTRIBUTING.md
• HACKING: https://github.com/simdjson/simdjson/blob/master/HACKING.md
• AI Usage Policy: https://github.com/simdjson/simdjson/blob/master/AI_USAGE_POLICY.md

If you can, we recommend running our tests with the sanitizers turned on.
For non-Visual Studio users, it is as easy as doing:

cmake -B build -D SIMDJSON_SANITIZE=ON -D SIMDJSON_DEVELOPER_MODE=ON
cmake --build build
ctest --test-dir build

Our CI checks, among other things, for trailing whitespace. If a test fails for that reason,
use the "artifacts" button to download the artifact and inspect the problematic lines,
or run scripts/remove_trailing_whitespace.sh locally if you have a bash shell and sed.

Checklist before submitting

• I added/updated tests covering my change (if applicable)
• Code builds locally and passes my check
• Documentation / README updated if needed
• Commits are atomic and messages are clear
• I linked the related issue (if applicable)

Final notes

• For large PRs, prefer smaller incremental PRs or request staged review.

Thanks for the contribution!