runtime: For c-archive/c-shared, install signal handlers synchronously.

runtime: For c-archive/c-shared, install signal handlers synchronously.

    This is a port of https://golang.org/cl/18150 to the gccgo runtime.

    The previous behaviour of installing the signal handlers in a separate
    thread meant that Go initialization raced with non-Go initialization if
    the non-Go initialization also wanted to install signal handlers.  Make
    installing signal handlers synchronous so that the process-wide behavior
    is predictable.

    Reviewed-on: https://go-review.googlesource.com/19494

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@233393 138bc75d-0d04-0410-961f-82ee72b054a4