The CC and CXX environmental variables cannot be set to the cross
compiler, meson expects them to point to the native compiler and the
build fails when meson tries to execute binaries compiled with CC and
CXX, as part of the configure tests (sanitycheckc.exe fails to
run). Unsetting CC and CXX means that clang and clang++ from the frida
sdk is used instead of the ones from the android-ndk. This is probably
fine for our purposes (and could otherwise be changed by patching
releng/setup-env.sh).
