Multiple tweaks to plat/linuxppc/descr

Multiple tweaks to plat/linuxppc/descr

Don't define __POWERPC.  I don't know any other compiler that defines
__POWERPC and don't want to invent a new macro.  Apple's gcc 4.0.1
from Xcode 2.5 defines __ppc__, _ARCH_PPC, __POWERPC__.  Debian's gcc
4.9.2-10 defines _ARCH_PPC, __PPC__, __powerpc__, __PPC, __powerpc,
PPC, powerpc.

Move the base vm address from 0x80000000 down to 0x10000000, as this
is where Debian loads /bin/true.  This is still higher than the base
addresses for linux386 and linux68k.

Sync led's arguments with linux386.