[AArch64] Fix OS ABI flag for aarch64-linux-gnu target.

For triple aarch64-linux-gnu we were incorrectly setting IRIX.
For triple aarch64 we are correctly setting SYSV.

Patch by Ana Pazos <apazos@codeaurora.org>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214974 91177308-0d34-0410-b5e6-96231b3b80d8