[Libomptarget][NFC] Remove use of VLA in the AMDGPU plugin (#69761)

Summary:
We should not rely on a VLA in C++ for the handling of this string. The
size is a true runtime value so we cannot rely on constexpr handling. We
simply use a small vector, whose default size is most likely large
enough to handle whatever size gets output within the stack, but is safe
in cases where it is not.