Import various libgcc functions from syslinux.

Experimentation reveals that gcc ignores -mrtd for the implicit
arithmetic functions (e.g. __udivdi3), but not for the implicit
memcpy() and memset() functions.  Mark the implicit arithmetic
functions with __attribute__((cdecl)) to compensate for this.

(Note: we cannot mark with with __cdecl, because we define __cdecl to
incorporate regparm(0) as well.)

src/libgcc/__umoddi3.c

@@ -0,0 +1,13 @@
+/*
+ * arch/i386/libgcc/__umoddi3.c
+ */
+
+#include "libgcc.h"
+
+LIBGCC uint64_t __umoddi3(uint64_t num, uint64_t den)
+{
+  uint64_t v;
+
+  (void) __udivmoddi4(num, den, &v);
+  return v;
+}