nds32: ELF definitions

This patch adds definitions for the ELF format, relocation types, vdso
locations and EXEC_PAGESIZE.

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>