Remove rtdebug_! and make rtdebug! work properly.

It now actually does logging, and is compiled out when `--cfg rtdebug` is not
given to the libstd build, which it isn't by default. This makes the rt
benchmarks 18-50% faster.
auto merge of #8419 : cmr/rust/fix-rtdebug, r=brson

It now actually does logging, and is compiled out when `--cfg rtdebug` is not
given to the libstd build, which it isn't by default. This makes the rt
benchmarks 18-50% faster.