Testsuite uses $(CC) which is not set by hadrian

There are numerous Makefile based tests in the testsuite that calls $(CC) to compile C sources; however, hadrian only sets TEST_CC environment variable when invoking the testsuite driver. This is troublesome when I'm testing a cross GHC that's been configured with a certain cross C compiler at configure time, but my shell's CC defaults to a host C compiler. We should make hadrian set CC and any other environment variable referred to by the Makefiles, or correct all $(CC) use sites to use $(TEST_CC).