rules.mak: introduce DSO rules

Add necessary rules and flags for shared object generation.
The new rules introduced here are:

1) %.o in $(common-obj-m) is compiled to %.o, then linked to %.so.

2) %.mo in $(common-obj-m) is the placeholder for %.so for pattern
matching in Makefile. It's linked to "-shared" with all its dependencies
(multiple *.o) as input. Which means the list of depended objects must
be specified in each sub-Makefile.objs:

    foo.mo-objs := bar.o baz.o qux.o

in the same style with foo.o-cflags and foo.o-libs. The objects here
will be prefixed with "$(obj)/" if it's a subdirectory Makefile.objs.

3) For all files ending up in %.so, the following is added automatically:

    foo.o-cflags += -fPIC -DBUILD_DSO

Also introduce --enable-modules in configure, the option will enable
support of shared object build. Otherwise objects are static linked to
executables.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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