[llvm] Add RISC-V support information to readme (PR #132699)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> As I understand, also for riscv32. Many teams are gathering under this architecture.

Can you add a line saying both `riscv64` and `riscv32` are supported?

https://github.com/llvm/llvm-project/pull/132699