Split TestGlobalVariables into two and xfail one of them for arm64 linux

Displaying of global pointer variables is not working on arm64 linux
(pr37301).

I've moved this part into a separate test, so it can be xfailed
separately. I then move the "show-variables-with-process-available"
check before the "show-all-variables" command to presrve the intent of
checking that global variable caching works correctly. (I've verified
that the new arrangement still fails when I revert the fix from
r331230.)

llvm-svn: 331250