Makefile: fix build when VPATH is outside GIT tree

Steve Ellcey / Leon Alrae reported that QEMU fails to build when
the VPATH directory is outside of the GIT tree, and the system
emulators & tools build is disabled. eg

   cd ..
   mkdir build
   cd build
   ../qemu/configure --disable-system --disable-tools
   make
   (...)
   make[1]: *** No rule to make target `../qom/object.o', needed by `qemu-aarch64'. Stop.
   make: *** [subdir-aarch64-linux-user] Error 2

The problem is due to the fact that some sub directory deps
were listed against SOFTMMU_SUBDIR_RULES instead of SUBDIR_RULES,
so were only processed for system emulators, not user emalutors.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1442570495-22029-1-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>