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.

tests/testincludes/std/stdarg.d

@@ -0,0 +1,22 @@
+
+/*
+ * Placed in public domain.
+ * Written by Hauke Duden and Walter Bright
+ */
+
+/* This is for use with variable argument lists with extern(D) linkage. */
+
+module std.stdarg;
+
+alias void* va_list;
+
+template va_arg(T)
+{
+    T va_arg(inout va_list _argptr)
+    {
+	T arg = *cast(T*)_argptr;
+	_argptr = _argptr + ((T.sizeof + int.sizeof - 1) & ~(int.sizeof - 1));
+	return arg;
+    }
+}
+