[compiler-rt] Fix compile error on ohos (PR #118193)

[Peng Huang via llvm-commits]

phuang wrote:

> Was there any explanation from the openharmony community why this function was missing?
> 

They told me this function is not exposed in ohos sdk, but they didn't explain why. I will ask them to explain it.

> Between fixing openharmony sdk vs your latest patch, do you still feel your patch is the right solution?
Another solution, I can test `get_current_dir_name()` in cmake script, and guard it with HAS_GET_CURRENT_DIR_NAME macro. what do you think? 


https://github.com/llvm/llvm-project/pull/118193