Scaladoc: Doctool doesn't include -scalajs flag to compile TASTY files of JSProject

[pikinier20]

This is summary bug report that covers #12215 and #11943. The reason of failures while documenting these projects is that Scaladoc tries to compile TASTY files but doesn't set -scalajs flag which seems to be mandatory while compiling JS projects.

There was a test made by me:

1. Set a basic sbt project with JS target.
2. Add minimized source proposed by @griggt in Dottydoc fails to read symbol in scalajs project on RC2 #11943.
3. Compile the project in order to generate .tasty file
4. Try to sbt doc this source <- Compilation error
5. Try to compile generated .tasty file with scalac with set -scalajs and with scalajs library on classpath <- Compilation success