[CMake] fix runpath for ELF platforms

[finagolfin]

Remove the absolute path to the host stdlib for libswiftDispatch.so and add $ORIGIN to it and libdispatch.so.

Otherwise, you see the following in the current official release for linux:

> readelf -d swift-5.2.3-RELEASE-ubuntu18.04/usr/lib/swift/linux/lib*ispatch.so | ag "File:|runpath"
File: swift-5.2.3-RELEASE-ubuntu18.04/usr/lib/swift/linux/libdispatch.so
File: swift-5.2.3-RELEASE-ubuntu18.04/usr/lib/swift/linux/libswiftDispatch.so
0x000000000000001d (RUNPATH)            Library runpath: [/home/buildnode/jenkins/workspace/oss-swift-5.2-package-linux-ubuntu-18_04/build/buildbot_linux/swift-linux-x86_64/lib/swift/linux]

[finagolfin]

@benlangmuir, you're one of two people to merge to this repo in the last 7 months, mind reviewing this tiny pull?

[benlangmuir]

@compnerd mind reviewing the cmake changes?

This isn't a project I work on, so I'll try to find someone who can approve this.

[MadCoder]

@swift-ci please test

[compnerd]

I think that it’s better to use the CMake variable to control the default RPATH.

[finagolfin]

Not sure what you mean, this pull does use the CMake INSTALL_RPATH variable.