UniCore32-bugfix: fix mismatch return value of __xchg_bad_pointer

When disintegrate system.h, I left an error in asm/cmpxchg.h, which
will result in following error:

arch/unicore32/include/asm/cmpxchg.h: In function '__xchg':
arch/unicore32/include/asm/cmpxchg.h:38: error: void value not ignored as it ought to be

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>