[sancov] Run more sancov tests on non-x86-Linux machines

Add the $arch-registered-target features that clang uses to disable
tests that require a registered backend, so that we can run the sancov
tests on Windows. LLVM's lit suite did not appear to have a per-test way
to do this, and I would rather not split up the sancov tests into
architecture directories.

Split out of https://reviews.llvm.org/D23321

llvm-svn: 278271