sw64: merge user_fpsimd_state into thread_struct

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5G7NL



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The user_fpsimd_state struct has been defined to hold fpu state
which is also held in thread_struct. To reuse this struct, this
patch makes a little change to user_fpsimd_state, then replace
struct context_fpregs and fpcr of thread_struct.

This patch also moves definition of task_pt_regs and mm_segment_t
to appropriate headers to avoid compile errors.

Signed-off-by: He Sheng <hesheng@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>