Fixed DStress tests nocompile/c/{const_32_B.d,const_32_C.d}. Updated the runtest script to build libtangobos-partial.a if it hasn't already been built. Added in signbit() and va_arg!()() to libtangobos-partial.a so more of the phobos dependent DStress tests pass.

module bug20;
extern(C) int printf(char*, ...);

void func(void delegate() dg)
{
    dg();
}

void main()
{
    int i = 42;
    void delegate() dg = {
        i++;
    };
    printf("i = %d\n",i);
    func(dg);
    printf("i = %d\n",i);
    assert(i == 43);
}