[flang] Rewrite test for CPU_TIME

Don't rely on volatile writes to keep the CPU busy - it seems MSVC
optimizes them out, so we don't get different values for 'start' and
'end' on Windows. Rewrite the test to loop until we get a different
value for 'end'.

Fix suggested by Michael Kruse in
https://reviews.llvm.org/rG57e85622bbdb2eb18cc03df2ea457019c58f6912#inline-6002

Committing to fix the Windows buildbot, post-commit comments welcome!