ghc-toolchain: Configure Cpp and HsCpp correctly when user specifies flags

In ghc-toolchain, we were only /not/ configuring required flags when the
user specified any flags at all for the  of the HsCpp and Cpp tools.

Otherwise, the linker takes into consideration the user specified flags
to determine whether to search for a better linker implementation, but
already configured the remaining GHC and platform-specific flags
regardless of the user options.

Other Tools consider the user options as a baseline for further
configuration (see `findProgram`), so #23689 is not applicable.

Closes #23689