runtime: make mmap return 0 instead of -1 on aix/ppc64
runtime: make mmap return 0 instead of -1 on aix/ppc64

Most of the platforms are returning 0 instead of -1 when mmap syscalls
is failing. This patch corrects it for AIX in order to fix
TestMmapErrorSign and to improve AIX compatibility.

Change-Id: I1dad88d0e69163ad55c504b2b4a997892fd876cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/174297
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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