runtime: factor out mStackIsSystemAllocated

Rather than repeat long lists of GOOS values, factor out the code that checks
if a runtime starts on a system allocated stack. Note that this adds aix to
one case, which appears to have been previously missed.

Change-Id: I5cecb0bb47dd79cde8d723e5a42ba541e43cbfff
Reviewed-on: https://go-review.googlesource.com/c/go/+/250179


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org>