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 bug77;
import tango.stdc.string;
void main()
{
    size_t len;
    void func2()
    {
        char* prefix = "";

        void func()
        {
        len = strlen(prefix);
        assert(len == 0);
        }

        func();
    }
    func2();
}