sw64: optimize instruction usage in fork routine

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PNDF



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Use 'call' instead of 'jmp' in ret_from_fork(), so the 'ret' in
schedule_tail() won't mess up branch prediction.

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