Re-introduces ELF core file support for Linux x86-64
  Usage: 'lldb a.out -c core'.
  TODO: FreeBSD support.
  TODO: Support for AVX registers.
  TODO: Refactor so that RegisterContextCore* don't inherit from classes that use ProcessMonitor
  to fix the build on OS/X.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186516 91177308-0d34-0410-b5e6-96231b3b80d8