nios2: Remove unnecessary #ifdef guards

__HAVE_ARCH_MEMMOVE and __HAVE_ARCH_MEMSET are unconditionally defined
for nios2, so there is no need to protect the function definitions of
memmove() and memset().

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>