[lldb] Fix cross compiling on macOS

When cross compiling, a separate nested cmake is spawned, for building
host code generation tools such as lldb-tblgen.

When cross compiling on macOS, the nested native build would trigger
the lldb check for libc++, if testing is enabled (which it is by default).
(Even if `LLDB_INCLUDE_TESTS=OFF` is set on the main build, it has to
be passed separately in `CROSS_TOOLCHAIN_FLAGS_NATIVE` to reach the
nested build.)

Skip this check when building the host tools when cross compiling, as
the user won't try to run tests in that nested build.

(Alternatively, we could consider disabling all the `*_INCLUDE_TESTS`
by default in the nested host tools build.)

Differential Revision: https://reviews.llvm.org/D126557