[zorg] [PowerPC] Limit number of threads to 20 on clang-ppc64le-rhel buildbot

Earlier, we reduced the number of threads to 64 but we were still seeing
sanitizer tests failing. We experimented with the number of threads and saw
20 threads working fine for the bot. This patch reduces the number of threads
from 64 to 20 on the clang-ppc64le-rhel buildbot.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D88746
1 file changed