linux(4): Microoptimize rt_sendsig(), convert signal mask once

On amd64 Linux saves the thread signal mask in both contexts, in the machine
dependent and in the machine independent. Both contexts are user accessible.
Convert the mask once, then copy it.

MFC after: 2 weeks

(cherry picked from commit c30a767c6fd6d3f19e897fb800513e75175249b9)