[compiler-rt] [RISCV][compiler-rt] Small fixes for __riscv_feature_bits (PR #99958)

[Philip Reames via llvm-commits]

preames wrote:

I've written some simple test cases for the __attribute__((constructor)) feature, and everything appears to work as I'd expect.  I didn't find any super obvious breakage.

https://github.com/llvm/llvm-project/pull/99958