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/__moddi3.c

@@ -0,0 +1,26 @@
+/*
+ * arch/i386/libgcc/__moddi3.c
+ */
+
+#include "libgcc.h"
+
+LIBGCC int64_t __moddi3(int64_t num, int64_t den)
+{
+  int minus = 0;
+  int64_t v;
+
+  if ( num < 0 ) {
+    num = -num;
+    minus = 1;
+  }
+  if ( den < 0 ) {
+    den = -den;
+    minus ^= 1;
+  }
+
+  (void) __udivmoddi4(num, den, (uint64_t *)&v);
+  if ( minus )
+    v = -v;
+
+  return v;
+}