[Libreoffice] linux.mk and ppc, arm, etc and gmake build

So, the new solenv/gbuild/platform/linux.mk only supports x86 and
x86_64, no ppc, no arm, etc. e.g. -DX86/-DX86_64 and, DLLPOSTFIX of
lx/li etc.

We factored out the older dmake ones a while ago, so its not too hard to
see the little bits that are different per-platform. I suggest that I
stick e.g. little linux-ppc.mk, linux-arm.mk etc files beside linux.mk
and have linux.mk include e.g. linux-$(CPUNAME).mk to recover building
on those platforms with the new gmake stuff. Does that make sense ?

C.