runtime: align ucontext_t argument to 16 byte boundary

runtime: align ucontext_t argument to 16 byte boundary

    Some systems, such as ia64 and PPC, require that a ucontext_t pointer
    passed to getcontext and friends be aligned to a 16-byte boundary.
    Currently the ucontext_t fields in the g structure are defined in Go,
    and Go has no way to ensure a 16-byte alignment for a struct field.
    The fields are currently represented by an array of unsafe.Pointer.
    Enforce the alignment by making the array larger, and picking an offset
    into the array that is 16-byte aligned.

    Reviewed-on: https://go-review.googlesource.com/28910

From-SVN: r240044