Fix fulltest make rule.

make fulltest compiles 8cc several times. It compiles 8cc using
the default comipler (gen1), compile 8cc using gen1 (gen2),
and then compile 8cc using gen2 (gen3). After that it verifies
gen2 and gen3 are identical.

I noticed that gen3 was not actually compiled using gen2 but
instead gen1. This patch fixes the bug.