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

(cherry picked from commit a723bb667e48b9ec55c3a262c9cc49e700e52989)