[fuzzer] Use puts() rather than printf() in CopyFileToErr()

CopyFileToErr() uses Printf("%s", ...) which fails with a negative size on
files >2Gb (Its path is through var-args wrappers to an unnecessary "%s"
expansion and subject to int overflows) Using puts() in place of printf()
bypasses this path and writes the string directly to stderr. This avoids the
present loss of data when a crashed worker has generated >2Gb of output.

rdar://99384640

Reviewed By: yln

Differential Revision: https://reviews.llvm.org/D146189