[llvm] [RISCV] Add more files to .github/CODEOWNERS (PR #65234)

[Alex Bradbury via llvm-commits]

asb wrote:

For me personally, `**/*RISCV ` matches what I want and I'm not sure the volume of e.g. RISC-V specific LLD or LLDB patches is enough to justify splitting it out into separate teams....but it looks like I'm going to be handling labeling of any RISC-V related patches on my email client-side anyway, so my personal preference is even less relevant!

This proposed PR is I think definitely an incremental improvement on the current patterns, so I wouldn't be opposed to merging it and moving discussion on the 'ideal' setup to elsewhere. Marking it as LGTM on that basis.

https://github.com/llvm/llvm-project/pull/65234