[ProcessWindows] Implement a RegisterContextWindows for x86.

This implements the skeleton of a RegisterContext for Windows.
In particular, this implements support only for x86 general purpose
registers.

After this patch, LLDB on Windows can perform basic debugging
operations in a single-threaded inferior process (breakpoint,
register inspection, frame select, unwinding, etc).

Differential Revision: http://reviews.llvm.org/D6322
Reviewed by: Greg Clayton

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222474 91177308-0d34-0410-b5e6-96231b3b80d8