Previously, when setting BUILDTYPE=Debug, `make` would build both
the release build and the debug build and use the release build
to run certain build steps before running the tests using the
debug build. This patch adds another DEBUG_ONLY switch to the
Makefile. When it's used in conjunction with BUILDTYPE=Debug
Node.js no longer builds the release build and instead would
just use the debug build to complete the build steps.

```
DEBUG_ONLY=1 BUILDTYPE=Debug BUILD_WITH=ninja make test-only
```