runtime: reduce stack usage for asmcgocall on PPC64
Define it as ABIInternal, so the result does not take space on
stack.
Also use R10 as a temporary register for arithmetics on SP, so it
is hidden from the assembler's SP delta calculation, which is
irrelevant anyway as we are on the system stack.
Change-Id: I8fed467601c19cad2d7afab26978246d15ce3147
Reviewed-on: https://go-review.googlesource.com/c/go/+/580918
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TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>