[dev.cc] runtime: add explicit siginfo.si_addr field

struct siginfo_t's si_addr field is part of a union.
Previously, we represented this union in Go using an opaque
byte array and accessed the si_addr field using unsafe (and
wrong on 386 and arm!) pointer arithmetic.  Since si_addr is
the only field we use from this union, this replaces the
opaque byte array with an explicit declaration of the si_addr
field and accesses it directly.

LGTM=minux, rsc
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/179970044