MFC r356356, r356358, r356422: replace gcclibs' libssp

r356356:
Provide libssp based on libc

For libssp.so, rebuild stack_protector.c with FORTIFY_SOURCE stubs that just
abort built into it.

For libssp_nonshared.a, steal stack_protector_compat.c from
^/lib/libc/secure and massage it to maintain that __stack_chk_fail_local
is a hidden symbol.

libssp is now built unconditionally regardless of {WITH,WITHOUT}_SSP in the
build environment, and the gcclibs version has been disconnected from the
build in favor of this one.

r356358:
libssp: fix FORTIFY_SOURCE stub declarations

The LSB 4.1 that I referenced omitted the varargs, and I failed to catch it.
The __vsnprintf_chk error was from just downright misreading the page. GCC6
caught all of these, but I had only tested GCC4.2.

r356422:
Update libssp paths in various Makefile.depend* files

I've been advised that the model that uses these are fairly resilient, but
we do know the proper path to use (or remove, in the case of ^/targets/...),
so go ahead and update them to reflect that.