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/__udivmoddi4.c

@@ -0,0 +1,32 @@
+#include "libgcc.h"
+
+LIBGCC uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *rem_p)
+{
+  uint64_t quot = 0, qbit = 1;
+
+  if ( den == 0 ) {
+    return 1/((unsigned)den); /* Intentional divide by zero, without
+				 triggering a compiler warning which
+				 would abort the build */
+  }
+
+  /* Left-justify denominator and count shift */
+  while ( (int64_t)den >= 0 ) {
+    den <<= 1;
+    qbit <<= 1;
+  }
+
+  while ( qbit ) {
+    if ( den <= num ) {
+      num -= den;
+      quot += qbit;
+    }
+    den >>= 1;
+    qbit >>= 1;
+  }
+
+  if ( rem_p )
+    *rem_p = num;
+
+  return quot;
+}