add a linker search path to account for changes in library paths on platforms without fat binaries

[lanza]

swiftlang/swift#14243 modifies the paths for built libraries to oldpath/arch/libXYZ. This change adds a search path to account for this.

Requires
swiftlang/swift#14243
swiftlang/swift#14835

[lanza]

@compnerd

[compnerd]

This was tested at swiftlang/swift#14835

[huonw]

Unfortunately it looks like this breaks testing: https://ci.swift.org/job/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/289/console

...
22:46:46 + popd
22:46:46 ~/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10
22:46:46 xctest: using gold linker
22:46:46 + /home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift-corelibs-xctest/build_script.py --swiftc=/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/buildbot_incremental/swift-linux-x86_64/bin/swiftc --build-dir=/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/buildbot_incremental/xctest-linux-x86_64 --foundation-build-dir=/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/buildbot_incremental/foundation-linux-x86_64/Foundation --libdispatch-src-dir=/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift-corelibs-libdispatch --libdispatch-build-dir=/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/buildbot_incremental/libdispatch-linux-x86_64 --release
22:46:46 Traceback (most recent call last):
22:46:46   File "/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift-corelibs-xctest/build_script.py", line 577, in <module>
22:46:46     main()
22:46:46   File "/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift-corelibs-xctest/build_script.py", line 573, in main
22:46:46     parsed_args.func(parsed_args)
22:46:46   File "/home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift-corelibs-xctest/build_script.py", line 159, in build
22:46:46     swift_build_dir = os.path.abspath(args.swift_build_dir)
22:46:46   File "/usr/lib/python2.7/posixpath.py", line 360, in abspath
22:46:46     if not isabs(path):
22:46:46   File "/usr/lib/python2.7/posixpath.py", line 54, in isabs
22:46:46     return s.startswith('/')
22:46:46 AttributeError: 'NoneType' object has no attribute 'startswith'
22:46:46 /home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
22:46:46 /home/buildnode/jenkins/workspace/oss-swift-5.0-incremental-RA-linux-ubuntu-16_10/swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
...

It'd be great to get this fixed as soon as possible. Thanks!

[lanza]

Hey @huonw. This PR depends upon swiftlang/swift#14835 and will run just fine alongside it.