linux(4): Properly restore the thread signal mask after signal delivery on i386

Replace sigframe sf_extramask by native sigset_t and use it to
store/restore the thread signal mask without conversion to/from
Linux signal mask.

Pointy hat to: dchagin
MFC after: 2 weeks

(cherry picked from commit 4a6c2d075da0c5105269e6edcbe57bf6aaa0a0ae)