Currently ghc-toolchain will not try to be clever and will use user specified flags exactly as they are provided. Considering that ghc-toolchain is responsible for configuring the flags required by GHC, and since we can't expect the user to know about whether they need or not to include e.g. `-Wl,--no-as-needed` in the flags they pass, `ghc-toolchain` should, on a per-tool-per-flag-basis, update the user specified configuration with GHC-toolchain-specific knowledge.