askCc should be using the linker, not the compiler

When GHCi tries to find a shared lib, it calls "gcc --print-file-name"
to ask gcc where to find it.  But since we're looking for libraries,
we're really using the linker here, not the C compiler, so we should be
respecting the values of -pgml and -optl rather than -pgmc and -optc.

Test Plan: validate

Reviewers: bgamari, niteria, austin, hvr, erikd

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3393