tcg-ppc64: Support the ELFv2 ABI

The new ELFv2 ABI, used by default on powerpc64le-linux hosts,
introduced some changes that are incompatible with code currently
generated by the ppc64 TGC target.  In particular, we no longer
use function descriptors.

This patch adds support for the ELFv2 ABI in the ppc64 TGC
function call and function prologue sequences.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>