[clang] Fix __builtin_vectorelements tests with REQUIRES (PR #69582)

[Lawrence Benson via cfe-commits]

lawben wrote:

@nikic Sorry for the mess :/ 

I took the `target=` approach from [here](https://llvm.org/docs/TestingGuide.html#constraining-test-execution). Following the current buildbot status, this also does not seem to crash. Does this mean the rests are still incorrect and are not being executed at all? My local setup has all targets enabled, so it's hard for me to reproduce this locally. 

If this is still incorrect, I'd kindly ask you to point out the correct way of doing this across all tests. I guess it would have `REQUIRES: aarch64-registered-target` for AArch64 and `REQUIRES: riscv64-registered-target` for RISCV?

https://github.com/llvm/llvm-project/pull/69582