Implement GDB integration for source level debugging of code JITed using

Implement GDB integration for source level debugging of code JITed using
the MCJIT execution engine.

The GDB JIT debugging integration support works by registering a loaded
object image with a pre-defined function that GDB will monitor if GDB
is attached. GDB integration support is implemented for ELF only at this
time. This integration requires GDB version 7.0 or newer.

Patch by Andy Kaylor!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154868 91177308-0d34-0410-b5e6-96231b3b80d8