[TCWG-614] Range extension thunks - LLD now uses synthetic sections for Thunks. ARM Thunks now have symbols and mapping symbols. As a side-effect we can now create local symbols in synthetic sections so I've added them to the ARM PLT sections.

[Misc] - Wrote 3 lines of lld 4.0 release notes for ARM - Some upstream patch review in rtdyld and ilp32 support in the assembler - Presented my personal Linaro objectives to the TCWG leads.

Plans for next week: - Complete Fosdem presentation, will be leaving early Friday to catch the Eurostar - Respond to any post commit review comments on Thunks - Start work on range extension thunks, the previous commits to use synthetic sections were pre-requisites to range extension thunks, but added little new user-visible functionality.

Planned absences: - Fosdem 2017 will need to leave early on Friday 4th Feb - Holiday 21st Feb to 1st March - Linaro Connect 6th to 10th March Euro LLVM 27-28 March (Linaro's request and cost-code)