Change linker message verbosity to `-v2` (re #7863)

With this change, the linker status logging output such as

    Loading package ghc-prim ... linking ... done.
    Loading package integer-gmp ... linking ... done.
    Loading package base ... linking ... done.

is suppressed unless verbosity level is `-v2` or higher. This is done
to reduce the compiler message noise when TH is involved, which can
reduce the visibiliy of compile warnings.

Reviewed By: ekmett, austin

Differential Revision: https://phabricator.haskell.org/D232