[CMake][llbuildSwift] fix runpath for ELF platforms

[finagolfin]

Remove the absolute path to the host toolchain's stdlib from libllbuildSwift.so.

Otherwise, you see the following in the latest official 5.3 snapshot release for linux:

> readelf -d swift-5.3-DEVELOPMENT-SNAPSHOT-2020-05-04-a-ubuntu18.04/usr/lib/swift/pm/llbuild/libllbuildSwift.so | ag runpath
0x000000000000001d (RUNPATH)            Library runpath: [/home/buildnode/jenkins/workspace/oss-swift-5.3-package-linux-ubuntu-18_04/build/buildbot_linux/swift-linux-x86_64/lib/swift/linux:$ORIGIN:$ORIGIN/../../linux]

[finagolfin]

Btw, I also just noticed this broken relative rpath in the official 5.2.3 release for linux:

> readelf -d swift-5.2.3-RELEASE-ubuntu18.04/usr/lib/swift/pm/llbuild/libllbuildSwift.so | ag runpath
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:$ORIGIN:$ORIGIN/../../$<LOWER_CASE:Linux>]

That may also be worth fixing separately in the 5.2 branch, though given that nobody noticed so far, maybe not.

[dmbryson]

@swift-ci please smoke test

[finagolfin]

OS X failure looks spurious as this pull only affects linux, which passes.

[dmbryson]

@swift-ci please smoke test osx

[dmbryson]

@swift-ci please smoke test osx

[dmbryson]

@swift-ci please smoke test