[mono-runtimes] Update @(MonoFacadeAssembly)

[jonpryor]

Fixes: #1879

Add the following Facade assemblies to @(MonoFacadeAssembly):

• System.Buffers.dll
• System.Memory.dll
• System.Net.Http.Rtc.dll
• System.Threading.Tasks.Extensions.dll

Note: list was reordered to be sorted as per sort(1), and
generated with the following shell code on macOS:

$ cd external/mono/mcs/class/Facades
$ for d in `find . -depth 1 -type d | grep -v 'netstandard\|System.Drawing.Primitives' | sort -f` ; do
  n=`basename "$d"`;
  echo "    <MonoFacadeAssembly Include=\"$n.dll\" />";
done | pbcopy

The netstandard and System.Drawing.Primitives directory are
excluded so that the comment and following @(MonoFacadeAssembly)
entries in ProfileAssemblies.projitems are still relevant.

[marek-safar]

Don't include System.Net.Http.Rtc that one is desktop only

[marek-safar]

For the shell script I'd suggest to get the list from the binary package Mono provides to XA

[jonpryor]

@marek-safar suggested:

For the shell script I'd suggest to get the list from the binary package Mono provides to XA

This is a good idea, but requires that we have a binary mono archive to use, which does not currently exist for the mono/2018-06 branch that we're using, and might not exist for mono/2018-08 either...

Thus, for now, we need a way to determine (and update!) the Facade assembly list that doesn't involve the binary mono archive, and unless you have a better idea, using the contents of mcs/class/Facades is the best I've got.

[marek-safar]

@jonpryor using binary package only changes the location where you run your script with. If 2018-06 does not use binary packages you will have same binary output in your local build of Mono under mcs/class/lib/monotouch/Facades getting it from there should be same as getting the list from the binary archive.

System.Net.Http.Rtc has been removed.