[compiler-rt] [compiler-rt] Use static buffers in setvbuf/setbuf/setbuffer tests (PR #185204)

[Mats Kindahl via llvm-commits]

mkindahl wrote:

These are just tests, so not a biggie, but I am working on a proposal to [add a checker for this](https://github.com/mkindahl/llvm-project/tree/fix/setvbuf-stack-buffer) so I stumbled over it. 

https://github.com/llvm/llvm-project/pull/185204