[sancov] Revert r271695 which broke all of the PPC bots.

Original commit message:
[sancov] Run sancov tests on more platforms

The only tests that need to be run on Linux are the ones that use C++
demangling. I'm assuming they will fail on Mac, since __cxa_demangle
there won't handle the non-double-underscore prefixed mangled names.

llvm-svn: 271763