Avoid adding duplicates to SRCS/OBJS/SOBJS/POBJS

This is a change in preparation for stopping to use lorder.sh (D26044) and
instead assume that we have a linker newer than ~1990. Without lorder.sh
duplicates end up being passed to the linker when building .so files and this
can result in duplicate symbol definition errors.

There is one minor change: libcompiler_rt.a will no longer provide
gcc_personality_v0 and instead we now only have it in libgcc_eh.a/libgcc_s.so.
This matches GCC's behaviour.

Reviewed By: emaste, cem
Differential Revision: https://reviews.freebsd.org/D26042