LoongArch: use 40 bits address space for user

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BWFP



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default to use 40 bits address space for user.

Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>
Change-Id: I23088f43e700ed9b4eb15df2cc9c58299a0582ff