Avoid redefining kotlin2js task in "ant jslib"

This helps to specify custom runtime for the newly built kotlin2js (which is
supposed to be different for the two bootstrapping steps). Also now it's not
required to launch ant-tools task before jslib to build JS library
Compile kotlin-runtime-minimal.jar, include into bootstrap compiler

Include minimal runtime into bootstrap compiler (step 1 of 2 in the build)
instead of the full runtime because full runtime intersects with compiler on
classes from core/ (descriptors, JDR, ...) and these classes are in fact of
different versions. This effectively means that reflection can't be used from
the compiler, otherwise bootstrap compiler will not link (in fact it will only
fail at runtime when/if the reflection usage is reached).
Experimental