Disintegrate asm/system.h for Unicore32 [based on ver #3, changed by gxt]

Disintegrate asm/system.h for Unicore32. (Compilation successful)
The implementation details are not changed, but only splitted.
BTW, some codestyles are adjusted.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>