extend bootstrap for PGO on windows

When building LLVM/LLD as part of a build that asks LLVM to generate profiles, e.g. when
doing PGO, cmake or clang-cl don't automatically link clang's profiler runtime in,
causing undefined reference errors at link-time.

We do that manually, by adding clang's resource library folder to the library search path:
- for LLVM itself, by extending the linker args that `rustc_llvm`'s build script
  uses, to avoid the linker errors when linking `rustc_driver`.
- for LLD, by extending cmake's linker flags during the LLD build step.

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