[release-branch.go1.9] runtime: use MAP_ANON in sigstack check
    
    MAP_ANON is the deprecated but more portable spelling of
    MAP_ANONYMOUS. Use MAP_ANON to un-break the Darwin 10.10 builder.
    
    Updates #22930.
    
    Change-Id: Iedd6232b94390b3b2a7423c45cdcb25c1a5b3323
    Reviewed-on: https://go-review.googlesource.com/88316
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>