sw64: force context reload without hmcall swpctx

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



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The hmcall swpctx is a bit heavyweight for context reload, and
we have provided some other hmcalls to do this:
  - wrfen: enable fpu.
  - wrptbr: update CSR:PTBR only.
  - load_mm: force update CSR:PTBR and CSR:UPN to switch mm.

For smp setup, hmcall swpctx is also heavy because boot CPU only
has to prepare stack pointer for secondary CPUs. So we remove the
tidle_pcb[], and take tidle_ksp[] to hold target idle task's ksp.
Each secondary CPU loads its ksp and update CSR:PTBR at boot time.

With this patch, most hmcall swpctx invocations are removed.

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