MFC r314054: 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.

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