Only compile RegisterContextWindows_x86.cpp for x86 host architecture.

This fixes compilation failures in the 64-bit build of LLDB on Windows.

Patch by Aidan Dodds

Differential Revision: http://reviews.llvm.org/D6704

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@224528 91177308-0d34-0410-b5e6-96231b3b80d8