src-release: Strip "-cvs" from GDB source dir and tarball.

The nightly snapshots we have been creating in the past did not
include the "-cvs" suffix at the end of the version number. Snapshot
packaging started breaking ever since GDB switched to using BFD's
version number. Things got partially fixed with the previous change
to this file, but the change missed the fact that the "-cvs" suffix
in the tarball name (Eg: gdb-7.6.50-20130816-cvs.tar) is undesirable.
This patch removes it.

ChangeLog:

        * src-release (VER): When using $(TOOL)/common/create-version.sh,
        strip the "-cvs" suffix from the version number if present.