Revert "PowerPC64 ELFv2 ABI: cases for in-register parameter passing, return values, and structs"

This reverts commit ba62bdf862560a85e2d637a0762d0e4fdd0db8b0.

Reverting as per discussion with Zoltan Varga (see PR 1409 [1]).
Some rework needed.

[1] https://github.com/mono/mono/pull/1409

Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>