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: NJiaxun Yang <jiaxun.yang@flygoat.com>

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@792 bbd45198-f89e-11dd-88c7-29a3b14d5316