Do not run gcount test on any *armv7l* (fixes test on llvm-clang-win-x-armv7l)

There was one buildbot (for llvm-clang-win-x-armv7l) that hadn't been
run for the initial revision [1].

  [1]: https://lab.llvm.org/buildbot#builders/60/builds/12118

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D150886