amd64: check that %cs and %ss values from ucontext fit into registers

[kib]

Summary

This change only checks that the values from the user-supplied context
are not truncated by C implicit type convertions.  The validity of the
segment selectors is still checked by hardware.

[markj]

sys/amd64/amd64/exec_machdep.c

275

These fields are register_t, which is signed.

sys/amd64/ia32/ia32_signal.c

217

... here, the fields are unsigned.

[kib]

Centralize and fix checks.

[markj]

sys/amd64/ia32/ia32_signal.c

721

This should be updated too?

[kib]

Missed check.