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Currently, Trusty OS/LK implemented FPU context switch in internal
thread switch but does not implement the proper mechanism for world
switch. This commit just simply saves/restores FPU registes in world
switch to prevent FPU context from being currupted when Trusty OS uses
VFP in its applications.

It should be noted that the macro *CTX_INCLUDE_FPREGS* must be defined
in trusty.mk if Trusty OS uses VFP
Signed-off-by: Aijun Sun <aijun.sun@spreadtrum.com>