Provide a substitute to load command of gdb.

For bare-metal targets, lldb was missing a command like 'load' in gdb
which can be used to create executable image on the target. This was
discussed in
http://lists.llvm.org/pipermail/lldb-dev/2016-December/011752.html

This commits adds an option to "target module load" command to provide
that functionality. It does not set the PC to entry address which will
be done separately.

Reviewed in https://reviews.llvm.org/D28804

llvm-svn: 292499