Move gmp/config.mk.in to config.mk.in, fix #14972

Here's how the rube goldberg machine triggered the old bug:

1. If you have a file gmp/config.mk.in, then Cabal will
create a generated file in $DIST/build/gmp/config.mk

2. When you attempt to load inplace integer-gmp via GHCi, it will
ask gcc (aka clang on OS X) for the file name of 'gmp', with
base directory set to $DIST/build

3. There is a folder named 'gmp', and so this folder is returned
as the 'library' for gmp

4. GHCi loadArchive chokes to death trying to open a library
that is actually a folder

This patch solves the problem by breaking the chain at (1): if we
don't put config.mk in a folder named gmp, NO PROBLEM.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Test Plan: validate

Reviewers: angerman, hvr, bgamari

Reviewed By: angerman

Subscribers: erikd, thomie, carter

GHC Trac Issues: #14972

Differential Revision: https://phabricator.haskell.org/D4552

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