xbuild: include a Microsoft.Portable.Common.targets file (for F#)

F# PCL projects expect this file to exist in:
$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.Common.targets

So in order to avoid pristine F# PCL projects (created from VS) fail
the build that file has to exist, and import the Core targets like
its CSharp counterpart (Microsoft.Portable.CSharp.targets) does.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=17906
[corlib] Improve flow of ExecutionContext (few tests taken from https://github.com/mono/mono/pull/864). Fixes #18121 and similar.
[system.net.htt[] Pass CancellationToken in GetAsync. Fixes #13281
Merge pull request #915 from knocte/master

xbuild: include a Microsoft.Portable.Common.targets file (for F#)
[sgen] Change the bridge interface to include richer type information on classes.

There is an additional axis to bridge processing than just if a type is a bridge or not.
It can be processing transparent or opaque.

We didn't make this distinction before and treated all types as transparent.

Opaque types means that bridge processing should stop at them. An opaque bridge, in
addition to that, means it should be reported in.

This change paves the way for bridge graph pruning.
[corlib] Fix culture name constants in CreateSpecificCultureFromNeutral to be all lower case
Update CLDR to version 2.4. Fixes #17615
[runtime] Make different kinds of delegate invokes use a different name.
[io-layer] Bionic doesn't have clock_nanosleep.
[corlib] Fix the mobile profile.
When a read only property and the type is an interface we can't serialize the type.

This change is released under the MIT license.
Add test to ensure using concrete List as readonly property still serializes

This change is released under the MIT license.
[bcl] Work around quadratic behavior in NunitLite's AreEquivalent () methods.
Merge pull request #940 from alistair/ilist_xml_serialization

ilist xml serialization
[corlib] Add .net compatible separators for es culture
[corlib] Update en-au pm/am designators
[interpreter] Size reduction
[jit] Change the sgen vector allocator il code so it can be AOTed.
[jit] Add an optimization to use the optimized delegate invoke trampolines in some special cases, since the corresponding delegate invoke wrappers cannot be AOTed. Fixes part of #17423.
[jit] Add support for --debug=casts to one cast codepath, factor out common code. Fixes #18280.