ghc-toolchain: Certain options should be added by ghc-toolchain regardless of user specified flags

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.