Share `libc` signal definitions with more platforms

I confirmed that these 3 platforms share the same signal definitions
from these sources.

Haiku: https://github.com/haiku/haiku/blob/master/headers/posix/signal.h
OpenBSD: https://github.com/openbsd/src/blob/master/sys/sys/signal.h
NetBSD: http://fxr.watson.org/fxr/source/sys/signal.h?v=NETBSD

core/c/libc/signal.odin

@@ -34,7 +34,7 @@ when ODIN_OS == .Windows {
 	SIGTERM :: 15
 }
 
-when ODIN_OS == .Linux || ODIN_OS == .FreeBSD {
+when ODIN_OS == .Linux || ODIN_OS == .FreeBSD || ODIN_OS == .Haiku || ODIN_OS == .OpenBSD || ODIN_OS == .NetBSD {
 	SIG_ERR  :: rawptr(~uintptr(0))
 	SIG_DFL  :: rawptr(uintptr(0))
 	SIG_IGN  :: rawptr(uintptr(1))