Comment # 6 on bug 936463 from Richard Biener

As for 2) yes - the simple "crosses" only provide the compiler itself, not the
runtime.  The runtime needs to be deployed from populating the sysroot.  Kernel
developers seem to be happy without the runtime library.

As for 1) not sure, it was supposed to work but I can't remember whether I
really tested assembling.  I can reproduce it with the 4.9 based cross armv6hl
compiler I happen to have installed.  We probably miss some magic configury
here or get confused by --enable-version-specific-runtime-libs - who knows.

It certainly finds the tools at build time via
--with-build-time-tools=/usr/%{binutils_os}/bin