sw64: access pt_regs with regoffsets where appropriate

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



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The struct pt_regs of sw64 is a bit weird because r16~r18, r29 and
r30 are saved at odd location by hmcode, which makes it complicated
to reference them with regno. To improve maintainability, we map
these regno(s) in an unified way.

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

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