[CSSPGO][llvm-profgen] Make extended binary the default output format

Make extended binary the default output format for CSSPGO. This avoids having to pass flag every time when generating profile. It also matches llvm-profdata where binary profile is the default (should we switch to extbinary as default for llvm-profdata?).

We plan to compress name table for context profile, which depends on the built-in compression of extbinary.

Differential Revision: https://reviews.llvm.org/D103650