Update shared libraries version info to differentiate against v7.6

Update shared libraries version info to differentiate against v7.6

There are no API/implementation changes in libcord and libgccpp against
release-7_6 for now, so the corresponding version info is made the same.

There some implementation changes in libgc (without API changes) against
release-7_6, so the version info is made not the same (although there
are no API changes, 4:x:2 is already in use/reserved for release-7_6).

* Makefile.am (LIBGC_VER_INFO): Change version info to 5:0:3
(release-7_6 is supposed to use 4:x:2).
* Makefile.am (LIBGCCPP_VER_INFO): Change version info to 4:0:3
(the same one as release-7_6 uses at present).
* cord/cord.am (LIBCORD_VER_INFO): Likewise.