[libc] Add fuchsia/io.h for TestLogger

[abrachet]

Summary

b3c696fb86 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.

[Caslyn]

LGTM - the failed build doesn't look related to this change.

libc/src/__support/OSUtil/fuchsia/io.h

+//===------------- Fuchsia implementation of IO utils -----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_OSUTIL_FUCHSIA_IO_H
+#define LLVM_LIBC_SRC_SUPPORT_OSUTIL_FUCHSIA_IO_H
+
+#ifndef LIBC_COPT_TEST_USE_FUCHSIA
+#error this file should only be used by tests
+#endif
+
+#include "src/__support/CPP/string_view.h"
+
+#include <zircon/sanitizer.h>
+
+namespace __llvm_libc {
+
+LIBC_INLINE void write_to_stderr(cpp::string_view msg) {
+  __sanitizer_log_write(msg.data(), msg.size());
+}
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_FUCHSIA_IO_H

libc/src/__support/OSUtil/io.h

 #define LLVM_LIBC_SRC_SUPPORT_OSUTIL_IO_H
 
 #include "src/__support/macros/properties/architectures.h"
 
 #if defined(LIBC_TARGET_ARCH_IS_GPU)
 #include "gpu/io.h"
 #elif defined(__unix__)
 #include "linux/io.h"
+#elif defined(__Fuchsia__)
+#include "fuchsia/io.h"
 #endif
 
 #endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_IO_H