This centralizes the conditional logic into openssl/base.h so that it
doesn't have to be repeated. The name |OPENSSL_PRINTF_FORMAT_FUNC| was
chosen in anticipation of eventually defining an
|OPENSSL_PRINTF_FORMAT_ARG| for MSVC-style parameter annotations.

Change-Id: I273e6eddd209e696dc9f82099008c35b6d477cdb
Reviewed-on: https://boringssl-review.googlesource.com/6909
Reviewed-by: David Benjamin <davidben@google.com>