Changes required to use newGenfacadeModel (dotnet/corefx#36126)

Changes required to use newGenfacadeModel (dotnet/corefx#36126)

* making corefx compatible with newGenfacadesTask

correcting the new runtime variable

updating genfacades version to latest

fixes uap build and removes extra seed ype preferences for uapaot

* adding build configuration, comments and updating genfacadwa version

* fixing build  for netcoreapaot

* removing netcoreappaot build from all configurations build

Commit migrated from https://github.com/dotnet/corefx/commit/52206f12e2a34b708f33304d9b076b4958fbaa5e