syscall, internal/syscall/unix: Fix getrandom, clone on sparc64

syscall, internal/syscall/unix: Fix getrandom, clone on sparc64

    Since sparc is a valid architecture, the name of
    getrandom_linux_sparc.go means that it will be ignored on sparc64,
    even though it's whitelisted with a +build line.

    On SPARC, clone has a unique return value convention which requires
    some inline assembly to convert it to the normal convention.

    Reviewed-on: https://go-review.googlesource.com/30873

From-SVN: r241051