[libcpu] Refine MIPS common code

MIPS common code was highly duplicated, This commit
is a attempt to clean-up and refine these code.

The context and exception handle flow is mostly identical
with Linux, but a notable difference is that when FPU enabled,
we save FP registers in stackframe unconditionally.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

libcpu/mips/common/mips.h

@@ -12,14 +12,12 @@
 #define _COMMON_MIPS_H_
 
 #include "mips_cfg.h"
-
+#include "ptrace.h"
 #include "mips_types.h"
-#include "mips_asm.h"
-#include "mips_def.h"
+#include "asm.h"
 #include "mips_regs.h"
 #include "mips_addrspace.h"
 #include "mips_cache.h"
-#include "mips_context.h"
-#include "mips_excpt.h"
+#include "exception.h"
 
 #endif /* _COMMON_MIPS_H_ */