[X86DomainReassignment] Hopefully fix buildbot failure

The Darwin build bot failed with:
```
llc -mcpu=skylake-avx512 -mtriple=x86_64-unknown-linux-gnu domain-reassignment-test.ll -o - | llvm-mc
--
Exit Code: 134

Command Output (stderr):
--
Assertion failed: (MAI->hasSingleParameterDotFile()), function EmitFileDirective, file lib/MC/MCAsmStreamer.cpp, line 1087.
```

Looks like this is because the `llvm-mc` command was missing a triple
directive and defaulting to MachO. Add the triple option.

llvm-svn: 332694