Driver: use the canonical library prefix on Windows

Unlike Unices, Windows does not use a library prefix.  Use the traditional
naming scheme even for Windows itanium environments.  This makes the builtins
behave more like the sanitisers as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224996 91177308-0d34-0410-b5e6-96231b3b80d8