libcompiler_rt: stop building with stack smashing protection

libcompiler_rt implements certain functions that clang and gcc emit
calls to as part of their codegen (e.g. for extended width math).  Build
it without stack smashing protection (SSP, -fstack-protector) in order
to support building binaries without SSP, especially the dynamic linker.
Besides, SSP is probably not very valuable in this library.

Reviewed by:	arichardson, dim, kib
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D27786

lib/libcompiler_rt/Makefile

@@ -4,6 +4,7 @@
 
 PACKAGE=	lib${LIB}
 LIB=		compiler_rt
+MK_SSP=		no
 NO_PIC=
 WARNS?=		2