std: Implement Clone/TotalEq for ProcessExit

It's useful for structures which use deriving(Clone, TotalEq), even though it's
implicitly copyable.

Closes #13047
auto merge of #13051 : alexcrichton/rust/issue-13047, r=thestinger

It's useful for structures which use deriving(Clone), even though it's
implicitly copyable.

Closes #13047