Features/ssp.mk: Do not pass -fstack-protector as LDFLAGS

This isn't really a linker flag and worked only because C/C++ compiler is
usually used as linker.

This is backed by my own study of LLVM's code as well as this reply from
Flang's dev: https://discourse.llvm.org/t/flang-working-as-linker/87133/5

Approved by: bapt (portmgr)
Differential Revision: https://reviews.freebsd.org/D51116