Add -gnu in aarch64 Linux's triple

[futurejones]

swift-package-manager builds for master-branch and swift-5.2-branch are currently failing on Linux Aarch64 with following error:-

File "/usr/lib/python2.7/distutils/file_util.py", line 110, in copy_file
    "can't copy '%s': doesn't exist or not a regular file" % src)
distutils.errors.DistutilsFileError: can't copy '/home/worksonarm_test/jenkins_slave/workspace/swift-master-aarch64/build/buildbot_linux/swiftpm-linux-aarch64/aarch64-unknown-linux-gnu/release/swift-build': doesn't exist or not a regular file

Due to the new swiftpm build methods Linux Aarch64 now requires the -gnu tag on the triple.

This was adding to x86_64 in #2353

This is currently causing the build failures on https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-16.04-aarch64/

This changed will also need to be applied to the swift-5.2-branch

[aciidgh]

@swift-ci smoke test

[aciidgh]

Thanks! Can you also make a PR to swift-5.2-branch?

[futurejones]

Added PR for swift-5.2-branch #2505