__div64_32(): make it overridable at compile time

Some architectures may want to override the default implementation
at compile time to do things inline.  For example, ARM uses a
non-standard calling convention for better efficiency in this case.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
index 408856a..163f779 100644
--- a/include/asm-generic/div64.h
+++ b/include/asm-generic/div64.h
@@ -194,7 +194,9 @@
 }
 #endif
 
+#ifndef __div64_32
 extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
+#endif
 
 /* The unnecessary pointer compare is there
  * to check for type safety (n must be 64bit)