runtime/internal/syscall: convert PPC64 Syscall6 to ABIInternal
runtime/internal/syscall: convert PPC64 Syscall6 to ABIInternal

This avoids a lot of stacking.

Change-Id: If5c5cf33335ffdcb7eecbd3f2db7858a415d817d
Reviewed-on: https://go-review.googlesource.com/c/go/+/443736
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Run-TryBot: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go