[clang] Fix armv7-quick build by hardcoding -triple=x86_64 in OOM test.

The test was added in 48bb5f4c and it
creates a very large array, which is too large for ARM. Making the array
smaller is not a good option, since we would need a very low ulimit and
could then hit it for other reasons.

Should fix the https://lab.llvm.org/buildbot/#/builders/171/builds/5851
failure.

Differential Revision: https://reviews.llvm.org/D113583