Adds extern "C" ints to the .cpp files that use RegisterTarget, as

Adds extern "C" ints to the .cpp files that use RegisterTarget, as
well as 2 files that use "Registrator"s. These are to be used by the
MSVC builds, as the Win32 linker does not include libs that are
otherwise unreferenced, even if global constructors in the lib have
side-effects.

Patch by Scott Graham!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59378 91177308-0d34-0410-b5e6-96231b3b80d8