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 mini.const1;

void* g = cast(void*)&foobar;

int foobar()
{
    return 42;
}

void main()
{
    auto fn = cast(int function())g;
    int i = fn();
    assert(i == 42);
}