This commit allows to cross-compiling aix/ppc64. The nosplit limit must
twice as large as on others platforms because of AIX syscalls.
The stack limit, especially stackGuardMultiplier, was set by cmd/dist
during the bootstrap and doesn't depend on GOOS/GOARCH target.

Fixes #29572

Change-Id: Id51e38885e1978d981aa9e14972eaec17294322e
Reviewed-on: https://go-review.googlesource.com/c/157117
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>