Provide InitializeAllTargets and InitializeNativeTarget functions in the

Provide InitializeAllTargets and InitializeNativeTarget functions in the
C bindings.  Change all the backend "Initialize" functions to have C linkage.
Change the "llvm/Config/Targets.def" header to use C-style comments to avoid
compile warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74026 91177308-0d34-0410-b5e6-96231b3b80d8