[libc] Use string_view for write_to_stderr

This patch makes use of `cpp::string_view` instead of `const char*` for `write_to_stderr`. This helps sending non null-terminated buffers such as a single character, `cpp::string_view` or `cpp::string`.
It also fizes the gpu version that had several bugs (See https://reviews.llvm.org/D145913#4236641).

Differential Revision: https://reviews.llvm.org/D147375