[llvm-profdata] Change default output format of llvm-profdata to ExtBinary

ExtBinary is compatible to, and more superior than Binary format, which is the current default output format. In the long run we are looking to only support ExtBinary format and Text format (for visual inspection), and drop Binary format as well. Since Binary format was the default, we expect many users are still using it, so let's change the default output format first, and hopefully the usage decreases over time

Reviewed By: davidxl, hoy

Differential Revision: https://reviews.llvm.org/D149700