[CMake] Don't pass CMAKE_C(XX)_COMPILER to the nested NATIVE build when cross compiling

Originally, the nested build was set up with the CMake command
`execute_process` which implicitly passes CC/CXX variables for
the configured compiler, which then was picked up by the nested
CMake. (This CMake behaviour, to implicitly pass such variables
is up for discussion and might change in the future; see
https://gitlab.kitware.com/cmake/cmake/-/issues/21378.)

How the nested cmake build is set up was changed in
aa7d6db5 / D40229 - the old behaviour
was brought along by manually passing
-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER} to the nested cmake
configuration. This was then later made optional in
f5f0fffe / D40947. But still,
the default if the user doesn't pass
CROSS_TOOLCHAIN_FLAGS_${target_name} (e.g. CROSS_TOOLCHAIN_FLAGS_NATIVE)
is to pass in the surrounding build's compiler - which usually
doesn't work, and is quite non-obvious to figure out.

Just drop the default passing of the outer compiler, when cross
compiling. This should avoid surprising cases of using the cross
compiler for the native build for essentially all new users trying
to cross compile, until they've discovered CROSS_TOOLCHAIN_FLAGS_NATIVE.

Keep passing these when not cross compiling, e.g. if building with
optimized tablegen.

This was already suggested at the end in D40229, but apparently never
acted upon.

Differential Revision: https://reviews.llvm.org/D126313