The _SigUnblock flag was appended to SIGSYS slot of runtime signal table
for Linux in https://go-review.googlesource.com/22202, but there is
still no concrete opinion on whether SIGSYS must be an unblocked signal
for runtime.

This change removes _SigUnblock flag from SIGSYS on Linux for
consistency in runtime signal handling and adds a reference to #15204 to
runtime signal table for FreeBSD.

Updates #15204.

Change-Id: I42992b1d852c2ab5dd37d6dbb481dba46929f665
Reviewed-on: https://go-review.googlesource.com/22537Reviewed-by: Ian Lance Taylor <iant@golang.org>