linux: fix sigaltstack on amd64

To determine whether to use alternate signal stack or not,
we need to use the native signal number, not the one translated
with bsd_to_linux_signal().

In practical terms, this fixes golang.

Reviewed By: dchagin
Fixes: 135dd0cab51
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D31298