Prepare for clang+mingw windows builds

Use $(prefix)CLANG_SUPPORTED from Soong to control whether to use clang
for host builds or not. It's fairly general, but I only expect it to be
used to so that Windows builds can be switched between GCC and Clang
with a single switch.

Also separate native-host-cross from native-host so that it's easier to
build all possible windows modules.

Bug: 69933068
Test: m native-host-cross
Test: switch clang on and off, grep w64.*g++ out/build-aosp_arm.ninja
Change-Id: I205c5bc6739b72a539ece935381107ec1dd5bd48