coreclr should use the system copy of libunwind for some builds

[omajid]

CoreCLR recently got a bundled copy of libunwind (dotnet/coreclr#17094). For source-build, we should look at whether we want portable builds or not (#302) and use that to decide whether we should use the bundled copy of libunwind or the system-installed copy (dotnet/coreclr#17164).

Effectively, once our copy of coreclr is updated, we should pass along -cmakeargs -DCLR_CMAKE_USE_SYSTEM_LIBUNWIND=1 to the coreclr build if PortableBuild is true.

cc @dleeapho @crummel @aslicerh

[dleeapho]

/cc @dseefeld

[omajid]

I opened up dotnet/coreclr#17250 to see if this can be done completely in coreclr itself.

[omajid]

I opened up dotnet/coreclr#17250 to see if this can be done completely in coreclr itself.

It looks like PortableBuild can not be directly associated with the second concept of UseSystemLibraries. It is possible for one to be true but not the other: dotnet/coreclr#17250 (comment)

I will open a PR here to implement the second option directly.

[omajid]

Closing since this was fixed with #391. #416 disables this for Windows.