sw64: switch to generic fork like system calls

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5GF7A



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We put thread's callee-saved registers in thread_struct, and put
exception callee-saved registers in pt_regs. For context switch,
it switches fpu state first, and then switches integer registers
in __switch_to.

Because callee-saved registers are always saved and restored, it
will cause a little overhead.

As a result, the special sw64 fork like system calls are removed.

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

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