Provide a compatibility alias __memcpy

For some reason, libelf uses the internal glibc alias __memcpy, which
doesn't exist in uClibc. Add a manual alias so that the build can
proceed with uClibc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/libelf/libelf.h
===================================================================
--- a/libelf/libelf.h
+++ b/libelf/libelf.h
@@ -34,6 +34,11 @@
 /* Get the ELF types.  */
 #include <elf.h>
 
+#ifndef _LIBC
+#ifndef __mempcpy
+#define __mempcpy mempcpy
+#endif
+#endif
 
 /* Known translation types.  */
 typedef enum