[CMake] fix plutil runpath for ELF platforms

[finagolfin]

Remove the absolute path to the host toolchain's stdlib from plutil, and add it as a CMake BUILD_PATH, so that it's removed upon installation.

The problem is that plutil on linux currently has this rpath:

> readelf -d swift-DEVELOPMENT-SNAPSHOT-2021-05-05-a-centos8/usr/bin/plutil |ag runpath
 0x000000000000001d (RUNPATH)            Library runpath: [/home/build-user/build/buildbot_linux/swift-linux-x86_64/lib/swift/linux]

I submitted this as part of #2783 earlier, but had to remove it because the linux CMake was too old and didn't support BUILD_RPATH then. With swiftlang/swift#37517 updating the linux CMake, this should work now.

[finagolfin]

@drexin, could you look at this?

[finagolfin]

@varungandhi-apple, mind running the CI here?

[drexin]

@swift-ci test

[finagolfin]

The linux CI reports this error:

CMake Error at Sources/Tools/plutil/CMakeLists.txt:18 (message):
  Error getting target info with

    `/home/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-Linux/buildbot_linux/swift-linux-x86_64/bin/swiftc -module-cache-path "/home/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-Linux/buildbot_linux/foundation-linux-x86_64/module-cache" -print-target-info`

  Error:

    <unknown>:0: error: unknown argument: '-module-cache-path "/home/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-Linux/buildbot_linux/foundation-linux-x86_64/module-cache"'

I tried running the same command with a recent official trunk snapshot build, no problem. Technically, the command doesn't need to pass that flag in through CMAKE_Swift_FLAGS, but I'm future-proofing this for swiftlang/swift#33724, where cross-compilation targets will be passed in that way.

I have no idea why this wouldn't work here and the same flag works when building the full Foundation, anyone know?

[finagolfin]

Oh, think I got it, fix incoming.

[finagolfin]

Alright, had to do with string quoting. I hadn't tested adding that flag first, did so now.

[drexin]

@swift-ci test

[finagolfin]

macOS CI failure is unrelated, linux CI passing means this works.

[finagolfin]

Ping, just needs the macOS CI run again, which this ELF pull doesn't affect, and can get this in.

[varungandhi-apple]

@swift-ci test macOS

[finagolfin]

Some memory issue on the macOS CI? Again unrelated.

[varungandhi-apple]

OK-ed by Dario, merged. Sorry about the delay.