It seems that configure picks the wrong multi-os dir:
phobos_toolexecdir = ${libdir}/gcc/${host_alias}
phobos_toolexeclibdir = /../lib64
toolexecdir = $(phobos_toolexecdir)
toolexeclibdir = $(phobos_toolexeclibdir)
Should have been /usr/lib/../lib64 (i.e. /usr/lib64),
but seems that it used the wrong variable for that ?
On non-multi targets, the multi-os-directory is "."
so that escaped this bug (due to different code path)