Exclude -flto when building *genassym.o

The build process generates *assym.h using nm from *genassym.o (which is
in turn created from *genassym.c).

When compiling with link-time optimization (LTO) using -flto, .o files
are LLVM bitcode, not ELF objects. This is not usable by genassym.sh,
so remove -flto from those ${CC} invocations.

Submitted by: George Rimar
Reviewed by: dim
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D9659