libssp: don't compile with -fstack-protector*

This similarly matches what we do in libc; compiling libssp with
-fstack-protector* is actively harmful.  For instance, if the canary ctor
ends up with a stack protector then it will trivially trigger a false
positive as the canary's being initialized.

This was noted by the reporter as irc/ircd-hybrid started crashing at start
after our libssp was MFC'd to stable/11, as its build will explicitly link
in libssp. On FreeBSD, this isn't necessary as SSP bits are included in
libc, but it should absolutely not trigger runtime breakage -- it does mean
that the canary will get initialized twice, but as this is happening early
on in application startup it should just be redundant work.

Reported by:	Tod McQuillin <devin@sevenlayer.studio>
MFC after:	3 days

lib/libssp/Makefile

@@ -17,4 +17,8 @@ SRCS=		stack_protector.c fortify_stubs.c
 
 CFLAGS.fortify_stubs.c=	-Wno-unused-parameter
 
+# Stack protection on libssp symbols should be considered harmful, as we may
+# be talking about, for example, the guard setup constructor.
+SSP_CFLAGS:=
+
 .include <bsd.lib.mk>