Make getcontext(2) work on i386. It needs a small wrapper in libc

otherwise the return from the syscall stub for getcontext will pop off
the return value for the caller to the getcontext stub and it will appear
as though the setcontext() syscall returned instead of the getcontext().
The same bug exists on amd64, a fix is coming there too.

The bug can be demonstrated with this test code fragment:
main()
{
        ucontext_t top;

        if (getcontext(&top) == 0) {
                write(2, "PING!\n", 6);
                /* Cause a return value of 1 from getcontext this time */
                top.uc_mcontext.mc_eax = 1;
                setcontext(&top);
                err(1, "setcontext() returned");
        }
        write(2, "PONG!\n", 6);
        _exit(0);
}

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