[SR-9347] XCTest and dispatch compiled with wrong linker settings [was: SwiftPM libBlocksRuntime.so.0 missing dependency]

Issue Description:

SwiftPM in swift development snapshots of 2018-07-21-a and later fails on a clean Ubuntu installation with:

/swift-DEVELOPMENT-SNAPSHOT-2018-07-21-a-ubuntu16.04/usr/bin/swift-package: error while loading shared libraries: libBlocksRuntime.so.0: cannot open shared object file: No such file or directory

It seems that installing libblocksruntime-dev resolves this, but this is not a dependency that was required for any previous versions of Swift.

It looks like this additional dependency was introduced in this PR: https://github.com/apple/swift-package-manager/pull/1631

Snapshots around 2018-10-19-a and later do seem to include /usr/lib/swift/linux/libBlocksRuntime.so - possibly as a result of https://github.com/apple/swift-corelibs-libdispatch/pull/362 - but there is no libBlocksRuntime.so.0 symlink to this. Creating a symlink from libBlocksRuntime.so -> libBlocksRuntime.so.0 seems to make things work, but I'll freely admit that I don't really know what I'm doing.

Start by reproducing the missing libBlocksRuntime.so.0 error on a clean Ubuntu 16.04 installation with the affected Swift development snapshot. Compare the bundled libBlocksRuntime.so and linker settings, then verify that SwiftPM, XCTest, and dispatch load successfully without a manually created symlink.