When running this test on an aarch64 machine, it fails:
```
/usr/bin/ld.gold: error: .../test/tools/gold/X86/Output/comdat-nodeduplicate.ll.tmp/ab.lto.o: incompatible target
```
Specify the elf_x86_64 emulation as all of the other gold plugin tests
do.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D107020
(cherry picked from commit 5060224d9eed8b8359ed5090bb7c577b8575e9e7)