[libc] Fix printing on the GPU when given a `cpp::string_ref`

The implementation of the test printing currently expects a null
terminated C-string. However, the `write_to_stderr` interface uses a
string view, which doesn't need to be null terminated. This patch
changes the printing interface to directly use `fwrite` instead rather
than relying on a null terminator.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149493