nds32: Fix the allmodconfig build. To make sure CONFIG_CPU_LITTLE_ENDIAN is default y

This way we can build kernel with CONFIG_CPU_LITTLE_ENDIAN=y. Build allmodconfig
and allnoconfig are available too. It also fixes the endian mismatch issue
because AFLAGS and LDFLAGS is not passed correctly.

Signed-off-by: Vincent Ren-Wei Chen <vincentc@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>