Fix 'make dist' broken since 35287878

In commit 35287878b9785efeaeb854afb6ee4509f5714d65:

    [xbuild] Reorganize xbuild targets/frameworks/bin data

Several files were shuffled around, but the EXTRA_DISTFILES was
not fully updated with the new file paths.

This patch is required to build a 3.2.7 tarball.
Include xbuild_4_0_dirs in dist; fixes 425fdc03

In commit 425fdc03fa38b2ee29c1cf88617ddb1580fde78f:

  [xbuild] Build & install MSBuild 12.0 assemblies

The net_3_5_only_dirs containing the Build classes were moved to
various xbuild_X_dirs variables, but an undefined $(xbuild_dirs)
variable was appended to DIST_ONLY_SUBDIRS.

This patch adds $(xbuild_4_0_dirs) instead.
Update mono-core.spec.in with xbuild reorganization
Fix warnings.
[corlib] WeakReference<T>::TryGetTarget should return true only for non null targets.
Merge pull request #880 from awinters-fvs/xbuild/make-dist-fix

Fix 'make dist' broken since 35287878
[runtime] Use a separate lock for the joinable_threads () hash table.
[llvm] Disable llvm instead of asserting in some cases. Fixes #17527.
[runtime] Use mono_thread_join () to wait for the finalizer thread to exit.
[runtime] Fix a shutdown leak.
[mcs] Release array initializer stack field at the end of initialization. Fixes #17494
[mcs] Fixes warning
[mcs] Report less internal errors. Fixes #17472
[mcs] Explicit user conversion from nullable types does not have to lift the result. Fixes #17469
[mcs] Improve daa862f17f7930e5618d1e46e09de46f95c667d4 commit
[runtime] Move the thread exit code to mono-threads from io-layer.
[runtime] Remove some references to gc_wrapper.h which are no longer needed.
[corlib] Decimal parsing with banker's rounding with non-zero digits beyond the thousandths digit. Fixes #17536
More tests
[corlib] Fixes our of range index access from a5fc91fc42a9a1edcc5ad9d6c656c9c9223e62a8