[build/Facades] Parallelize the Facades directory build

The parallelization is done by overriding the default "all" target
and instead of using the serial subdirectory processing that goes
over the SUBDIR and xx_SUBDIR rules, we turn the list of
subdirectories into the name of the target, for example:

     System.Collections.Concurrent

becomes:

     ../../class/lib/net_4_5/Facades/System.Collections.Concurrent.dll

Then a pattern is added to handle that rule:

     $(target_dir)/%.dll:

This allows the build to be parallized.