[release-branch.go1.9] runtime: restore the Go-allocated signal stack in unminit
    
    Currently, when we minit on a thread that already has an alternate
    signal stack (e.g., because the M was an extram being used for a cgo
    callback, or to handle a signal on a C thread, or because the
    platform's libc always allocates a signal stack like on Android), we
    simply drop the Go-allocated gsignal stack on the floor.
    
    This is a problem for Ms on the extram list because those Ms may later
    be reused for a different thread that may not have its own alternate
    signal stack. On tip, this manifests as a crash in sigaltstack because
    we clear the gsignal stack bounds in unminit and later try to use
    those cleared bounds when we re-minit that M. On 1.9 and earlier, we
    didn't clear the bounds, so this manifests as running more than one
    signal handler on the same signal stack, which could lead to arbitrary
    memory corruption.
    
    This CL fixes this problem by saving the Go-allocated gsignal stack in
    a new field in the m struct when overwriting it with a system-provided
    signal stack, and then restoring the original gsignal stack in
    unminit.
    
    This CL is designed to be easy to back-port to 1.9. It won't quite
    cherry-pick cleanly, but it should be sufficient to simply ignore the
    change in mexit (which didn't exist in 1.9).
    
    Now that we always have a place to stash the original signal stack in
    the m struct, there are some simplifications we can make to the signal
    stack handling. We'll do those in a later CL.
    
    Fixes #22930.
    
    Change-Id: I55c5a6dd9d97532f131146afdef0b216e1433054
    Reviewed-on: https://go-review.googlesource.com/88315
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>