linux(4): Add a dedicated linux_exec_copyin_args()

[dchagin]

Summary

Because Linux allows to exec binaries with 0 argc.

MFC after: 1 week

[emaste]

Are we aware of any actual Linux binary use cases for argc=0? (I don't object to this change, just wondering.)

[dchagin]

In D40148#914234, @emaste wrote:

Are we aware of any actual Linux binary use cases for argc=0? (I don't object to this change, just wondering.)

well, I dont know, I’m not use Linux since 1997:)
I try to fix Glibc posix_spawn(), for which some glibc tests fails due to argc=0, also see comment in the linux_exec_copyin_args() explaining this change