LoongArch: Remove useless header compiler.h

The content of LoongArch's compiler.h is trivial, with some unused
anywhere, so inline the definitions and remove the header.

Signed-off-by: Jun Yi <yijun@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add exception/interrupt handling

Add the exception and interrupt handling machanism for basic LoongArch
support.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>