Actually default rustc to have debug! output

Turns out that even if the default is "enabled", that doesn't mean that the
CFG_ENABLE_DEBUG variable will be defined. Instead, test whether
CFG_DISABLE_DEBUG is defined and disable debug things if that's the case.
auto merge of #9385 : alexcrichton/rust/actually-ndebug, r=pnkfelix

Turns out that even if the default is "enabled", that doesn't mean that the
CFG_ENABLE_DEBUG variable will be defined. Instead, test whether
CFG_DISABLE_DEBUG is defined and disable debug things if that's the case.