cmd/internal/sys, cmd/dist, misc/cgo/testcshared: enable c-shared feature and test on loong64
Linux kernel on loong64 has no Dup2 syscall support, so we use Dup3 to replace it like arm64 and riscv64.
Updates #53301
Fixes #58784
Change-Id: I4e0be140a71b86f4626ed39d76cf3ac78f842018
Reviewed-on: https://go-review.googlesource.com/c/go/+/425478
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: WANG Xuerui <git@xen0n.name> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Run-TryBot: Ian Lance Taylor <iant@google.com>
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