re-parallelize Win32 build after Unicode::Normalize got XS

re-parallelize Win32 build after Unicode::Normalize got XS

commit 57dca39807 serialized the 1st and 3rd longest targets (Extensions
and UNIDATAFILES aka mktables) together after commit c6b7cc2176 which
upgraded Unicode::Normalize, and  Unicode::Normalize went from PP->XS,
and the XS version requires unicore/CombiningClass.pl in
cpan/Unicode-Normalize/mkheader during U::N's building. The serialization
added about 32 second to a "dmake -P8 all" on a 8 core machine. Fix this
by spltting off U::N from Extensions and put it in its own target. Making
Extensions, the longest target slightly shorter by removing 1 module
from its workload is another plus. See perl #126564 for time savings details.

[Committer filled in RT ticket number in commit message.]