MIPS: Move unaligned load/store helpers to inst.h

Move unaligned load/store helpers from unaligned.c to inst.h, then
other parts of the kernel can use these helpers.

Use __ASSEMBLY__ to guard the definition of "LONG" in asm.h to avoid
build error on IPxx platforms.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Pei Huang <huangpei@loongson.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>