cmd/internal/obj/ppc64: generate prologue code compatible with new ABI

This changes the ppc64 prologue to avoid clobbering the registers
that could contain incoming argument values. This means preserving
the values in R3 - R10 and R14 - R19 for ppc64.

Instead of modifying R3, R4, R5 and R6 the registers R22, R23, R24
and R25 are used.

The argument registers that could be clobbered by the call to
morestack are saved and restored around that call.

Change-Id: If354c3dc73f2c8537ef4e513e5a4c05d7bd22866
Reviewed-on: https://go-review.googlesource.com/c/go/+/343872


Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>