Adding a RegisterContextMinidump_x86_64 converter

Summary:
This is a register context converter from Minidump to Linux reg context.
This knows the layout of the register context in the Minidump file
(which is the same as in Windows FYI) and as a result emits a binary data
buffer that matches the Linux register context binary layout.
This way we can reuse the existing RegisterContextLinux_x86_64 and
RegisterContextCorePOSIX_x86_64 classes.

Reviewers: labath, zturner

Subscribers: beanz, mgorny, lldb-commits, amccarth

Differential Revision: https://reviews.llvm.org/D24919

llvm-svn: 282529