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/libgcc.h

@@ -0,0 +1,26 @@
+#ifndef _LIBGCC_H
+#define _LIBGCC_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+/*
+ * It seems as though gcc expects its implicit arithmetic functions to
+ * be cdecl, even if -mrtd is specified.  This is somewhat
+ * inconsistent; for example, if -mregparm=3 is used then the implicit
+ * functions do become regparm(3).
+ *
+ * The implicit calls to memcpy() and memset() which gcc can generate
+ * do not seem to have this inconsistency; -mregparm and -mrtd affect
+ * them in the same way as any other function.
+ *
+ */
+#define LIBGCC __attribute__ (( cdecl ))
+
+extern LIBGCC uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *rem);
+extern LIBGCC uint64_t __udivdi3(uint64_t num, uint64_t den);
+extern LIBGCC uint64_t __umoddi3(uint64_t num, uint64_t den);
+extern LIBGCC int64_t __divdi3(int64_t num, int64_t den);
+extern LIBGCC int64_t __moddi3(int64_t num, int64_t den);
+
+#endif /* _LIBGCC_H */