runtime: move libcall to stack for syscall package on aix/ppc64
runtime: move libcall to stack for syscall package on aix/ppc64

Inside syscall_syscall6 function, libcall can directly be on the stack.
This is first function called with //go:nosplit, unlike runtime syscalls
which can be called during the sigtramp or by others //go:nosplit
functions.

Change-Id: Icc28def1a63e525850ec3bfb8184b995dfeaa736
Reviewed-on: https://go-review.googlesource.com/c/go/+/171338
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,golang/go,golang/go,CAFxX/go,golang/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,golang/go,akutz/go,akutz/go,nawawi/go,josharian/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,josharian/go,golang/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,cloudflare/go,cloudflare/go,josharian/go,CAFxX/go