[test] Fix tools/gold/X86/comdat-nodeduplicate.ll on non-X86 hosts

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