We apparently never tested compiling from_cpython files
with gcc, unless you manually passed USE_CLANG=0 which I
doubt anyone did.

Add a pyston_gcc target that just defers to cmake's better
ability to use different compilers, and run the test suite
through that rather than just pyston_prof.  This gives us
better coverage and also allows the Makefile to fully defer
`make check` to cmake.

Also, fix a couple bugs that gcc exposed.