testsuite: fix up T11223's Makefile
As reported by Simon on ghc-devs, this causes the build on Windows
to fail because it cannot find the cc command. The Makefile here
actually already sets GCC=gcc, but for some reason then uses both
$(GCC) and $(CC) to refer to C compilation.
Signed-off-by: Austin Seipp <email@example.com>
Reviewed By: thomie
Differential Revision: https://phabricator.haskell.org/D2158