MERGED: Add a mutex around stg_sig_install

commit 8792391e
Author: Simon Marlow <marlowsd@gmail.com>
Date:   Mon Dec 12 09:29:56 2011 +0000

    Add a mutex around stg_sig_install

    Protects against a race when two threads call installHandler
    simultaneously.  This was causing occasional failure of the test
    libraries/process/tests/3231(threaded2).