Now LibreOffice is availalbe in sisyphus_riscv64,
let's use it like all the other platforms do.

Note that LibreOffice-still is not yet available
in sisyphus_riscv64, so riscv64 joins e2k in
selecting LibreOffice for use/office/LibreOffice/still.

There are literally no office packages on riscv64 so far,
let's ease the need to count that in within pkglists like
it was done in slinux/misc-base since this approach makes
for unreadable conditional chunks if we look at LO-still
being absent on e2k; a common place should be in place.

Added use/office/LibreOffice/lang/extra target inspired
by pkg.in/lists/slinux/misc-base as well as targets for
two more LO subpackages; and use/office/LibreOffice/full
so that distros don't have to reiterate on all of those
particularities.

There's no LibreOffice-still on e2k (no 6.x for now,
just a single production 5.x build); and there's no
office (neither LO nor abiword/gnumeric) at all on
riscv64 as of today.  These nuances shouldn't bother
release managers each time but should lurk under
this feature's cover.

This feature ensures the availability and specification of the choice of
office package by analogy with the choice of browser. Additional options
may be set by explicit or indirect request.