mac: Make ubsan test config look more like asan test config

In particular, don't call get_target_flags_for_arch() since that
will cause an error in some situations:

If DARWIN_iossim_ARCHS=i386;x86_64, DARWIN_osx_ARCHS=x86_64, and
DARWIN_iossym_SYSROOT isn't set (due to the simulator sysroot not being
available), then config-ix.cmake won't add i386 to COMPILER_RT_SUPPORTED_ARCH
but ubsan's test/CMakeLists.txt would call get_target_flags_for_arch()
with i386, which would then run into the error in
get_target_flags_for_arch().

Having these conditions isn't ideal. The background here is that we
configure our mac-hosted trunk bots all the same (so they all have the
same DARWIN_*_archs, and we don't easily know if a mac host bot is
targeting mac or ios at the place where we call cmake), but only the
ios-targeting bots have ios sysroots available.

This will hopefully unbreak that use case without impacting anything
else -- and it makes ubsan and asan test setup more alike.

llvm-svn: 362010