elf-core: Split up parsing code into os-specific functions

Summary:
We've had a single function responsible for splitting a core segment
into notes, and parsing the notes themselves, bearing in mind variations
between 4 supported OS types. This commit splits that code into 5
pieces:
- (os-independent) code for splitting a segment into individual notes
- per-os function for parsing the notes into thread information

Reviewers: clayborg, krytarowski, emaste, alexandreyy, kettenis

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D40311

llvm-svn: 318903