Don't build sanitizer runtimes under WITHOUT_CXX

In the past we built the sanitizer runtimes when building Clang
(and using Clang as the compiler) but 7676b388adbc changed this to
be conditional only on using Clang, to make the runtimes available
for external Clang.

They fail to build when WITHOUT_CXX is set though, so add MK_CXX
as part of the condition.

Reported by: Michael Dexter, Build Option Survey
Reviewed by: imp, jrtc27
Fixes: 7676b388adbc ("Always build the sanitizer runtimes...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32731