[JITLoaderGDB] Read jit entry struct manually.

Summary:
Though r264012 was fancy enough to make reading the jit entry struct
work with templates, the packing and alignment attributes do not work on
Windows. So, this change makes it plain and simple with manual reading
of the jit entry struct.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18379

llvm-svn: 264217