build: add DEBUG_ONLY to Makefile to support debug-only builds

[joyeecheung]

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

[joyeecheung]

Draft for now, I've only tested it with the ninja builds, not the make builds

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/53429/

[bnoordhuis]

Not that I have a super strong opinion but... I just make -j$(getconf _NPROCESSORS_ONLN) -C out BUILDTYPE=Debug. Maybe document that somewhere instead of hacking up the Makefile even further? It's already quite illegible and impenetrable.

[joyeecheung]

Maybe document that somewhere instead of hacking up the Makefile even further?

The point is not just building debug builds, but also running all the tests with just the debug builds i.e. after running make test, out/Release is empty. Otherwise it just doubles the time it takes to run and test the debug builds if your machine already runs the debug build fast enough for the build steps of tests. (Personally, I always do ninja -C out/Debug, but then I still had to wait a long time to build the release build if I want to run the test suite with the debug build, which makes..little sense)