[libc] Add fuchsia/io.h for TestLogger

b3c696fb started using TestLogger in tests. This class depends on
write_to_stderr which was previously only provided for Linux and
GPU's. Note, this function is used on those platforms in the libc
proper, though for Fuchsia we only ever want to provide this for
TestLogger.

No CMake files are changed because this fuchsia/ directory isn't
useful in any CMake build.

Differential Revision: https://reviews.llvm.org/D148675