runtime: fix sigaction struct on freebsd

Fix the sa_mask member of the sigaction struct - on FreeBSD this is
declared as a sigset_t, which is an array of four unsigned ints.
Replace the current int64 with Sigset from defs_freebsd_GOARCH, which
has the correct definition.

Unbreaks the FreeBSD builds.

R=golang-dev, dave, minux.ma
CC=golang-dev
https://codereview.appspot.com/7333047