[SampleFDO] Add ExtBinary format to support extension of binary profile.

[SampleFDO] Add ExtBinary format to support extension of binary profile.

This is a patch split from https://reviews.llvm.org/D66374. It tries to add
a new format of profile called ExtBinary. The format adds a section header
table to the profile and organize the profile in sections, so the future
extension like adding a new section or extending an existing section will be
easier while keeping backward compatiblity feasible.

Differential Revision: https://reviews.llvm.org/D66513

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369798 91177308-0d34-0410-b5e6-96231b3b80d8